Having played second fiddle to Alexis Sanchez for much of the campaign, injuries to Theo Walcott and Alex Oxlade-Chamberlain gave Giroud a chance in Arsene Wenger's starting line-up.

On what was looking like a frustrating afternoon as the Gunners struggled to break down the West Brom rearguard, who had goalkeeper Ben Foster in particularly fine form, Giroud headed home an 87th-minute effort to earn a 1-0 victory.

The win keeps Arsenal within sight of London rivals Chelsea, who are nine points clear of Wenger's men at the league summit, and defender Koscielny believes Giroud's temperament while being on the sidelines has been exemplary.

"He stayed focused with the team," France international Koscielny told arsenal.com.

"We are not 11 or 18 players, we are 25 players. All the players who play for Arsenal have a big part to play during the season because of injuries and suspensions, and everyone needs to be ready to fight for this club.

"Olivier started and he tried to give his best on the pitch. He fought and it's good for him to score the winning goal.

"It was so important because last week we lost two games in seven days and we wanted to get back to winning three points.

"It was a hard game because West Brom play with their qualities, which include making big blocks, and that made it hard for us to find a good solution."

Arsenal vs West Brom player ratings Show all 22 1 /22 Arsenal vs West Brom player ratings Arsenal vs West Brom player ratings Petr Cech - 7 out of 10 LONDON, ENGLAND - SEPTEMBER 25: Jay Rodriguez of West Bromwich Albion shoots past Petr Cech of Arsenal during the Premier League match between Arsenal and West Bromwich Albion at Emirates Stadium on September 25, 2017 in London, England. (Photo by Michael Steele/Getty Images) Getty Images Arsenal vs West Brom player ratings Laurent Koscielny - 7 LONDON, ENGLAND - SEPTEMBER 25: Laurent Koscielny of Arsenal is faced by Hal Robson-Kanu and Kieran Gibbs of West Bromwich Albion during the Premier League match between Arsenal and West Bromwich Albion at Emirates Stadium on September 25, 2017 in London, England. (Photo by Mike Hewitt/Getty Images) Getty Images Arsenal vs West Brom player ratings Shkodran Mustafi - 7 LONDON, ENGLAND - SEPTEMBER 25: Hal Robson-Kanu of West Bromwich Albion and Shkodran Mustafi of Arsenal battle for the ball during the Premier League match between Arsenal and West Bromwich Albion at Emirates Stadium on September 25, 2017 in London, England. (Photo by Michael Steele/Getty Images) Getty Images Arsenal vs West Brom player ratings Nacho Monreal - 8 LONDON, ENGLAND - SEPTEMBER 25: Nacho Monreal of Arsenal clears the ball from the goal line during the Premier League match between Arsenal and West Bromwich Albion at Emirates Stadium on September 25, 2017 in London, England. (Photo by Michael Steele/Getty Images) Getty Images Arsenal vs West Brom player ratings Hector Bellerin - 7 LONDON, ENGLAND - SEPTEMBER 25: Tony Pulis, Manager of West Bromwich Albion shouts as Hector Bellerin of Arsenal take a throw in during the Premier League match between Arsenal and West Bromwich Albion at Emirates Stadium on September 25, 2017 in London, England. (Photo by Mike Hewitt/Getty Images) Getty Images Arsenal vs West Brom player ratings Mohamed Elneny - 6 West Bromwich Albion's English midfielder Gareth Barry (L) vies with Arsenal's Egyptian midfielder Mohamed Elneny during the English Premier League football match between Arsenal and West Bromwich Albion at the Emirates Stadium in London on September 25, 2017. / AFP PHOTO / Ian KINGTON / RESTRICTED TO EDITORIAL USE. No use with unauthorized audio, video, data, fixture lists, club/league logos or 'live' services. Online in-match use limited to 75 images, no video emulation. No use in betting, games or single club/league/player publications. / (Photo credit should read IAN KINGTON/AFP/Getty Images) AFP/Getty Images Arsenal vs West Brom player ratings Granit Xhaka - 6 LONDON, ENGLAND - SEPTEMBER 17: Granit Xhaka of Arsenal during the Premier League match between Chelsea and Arsenal at Stamford Bridge on September 17, 2017 in London, England. (Photo by David Price/Arsenal FC via Getty Images) Arsenal FC via Getty Images Arsenal vs West Brom player ratings Sead Kolasinac - 7 LONDON, ENGLAND - SEPTEMBER 14: Sead Kolasinac of Arsenal controls the ball during the UEFA Europa League group H match between Arsenal FC and 1. FC Koeln at Emirates Stadium on September 14, 2017 in London, United Kingdom. (Photo by Dan Mullan/Getty Images) Getty Images Arsenal vs West Brom player ratings Aaron Ramsey - 7 LONDON, ENGLAND - SEPTEMBER 25: Aaron Ramsey of Arsenal is challenged by goalkeeper Ben Foster of West Bromwich Albion during the Premier League match between Arsenal and West Bromwich Albion at Emirates Stadium on September 25, 2017 in London, England. (Photo by Mike Hewitt/Getty Images) Getty Images Arsenal vs West Brom player ratings Alexis Sanchez - 8 Arsenal's Chilean striker Alexis Sanchez reacts during the English Premier League football match between Arsenal and West Bromwich Albion at the Emirates Stadium in London on September 25, 2017. / AFP PHOTO / Ian KINGTON / RESTRICTED TO EDITORIAL USE. No use with unauthorized audio, video, data, fixture lists, club/league logos or 'live' services. Online in-match use limited to 75 images, no video emulation. No use in betting, games or single club/league/player publications. / (Photo credit should read IAN KINGTON/AFP/Getty Images) AFP/Getty Images Arsenal vs West Brom player ratings Alexandre Lacazette - 9 LONDON, ENGLAND - SEPTEMBER 25: Alexandre Lacazette of Arsenal heads past Ben Foster of West Bromwich Albion as he scores their first goal during the Premier League match between Arsenal and West Bromwich Albion at Emirates Stadium on September 25, 2017 in London, England. (Photo by Mike Hewitt/Getty Images) Getty Images Arsenal vs West Brom player ratings Ben Foster - 6 LONDON, ENGLAND - SEPTEMBER 25: Aaron Ramsey of Arsenal is challenged by goalkeeper Ben Foster of West Bromwich Albion during the Premier League match between Arsenal and West Bromwich Albion at Emirates Stadium on September 25, 2017 in London, England. (Photo by Mike Hewitt/Getty Images) Getty Images Arsenal vs West Brom player ratings Craig Dawson - 5 BURTON-UPON-TRENT, ENGLAND - JULY 26: Craig Dawson of West Bromwich Albion looks on during the pre season friendly match between Burton Albion and West Bromwich Albion at Pirelli Stadium on July 26, 2017 in Burton-upon-Trent, England. (Photo by Nathan Stirk/Getty Images) Getty Images Arsenal vs West Brom player ratings Ahmed El-Sayed Hegazi - 7 LONDON, ENGLAND - SEPTEMBER 25: Alexis Sanchez of Arsenal and Ahmed El-Sayed Hegazi of West Bromwich Albion chase the ball during the Premier League match between Arsenal and West Bromwich Albion at Emirates Stadium on September 25, 2017 in London, England. (Photo by Mike Hewitt/Getty Images) Getty Images Arsenal vs West Brom player ratings Jonny Evans - 6 LONDON, ENGLAND - SEPTEMBER 25: Nacho Monreal of Arsenal takes on Jonny Evans of West Bromwich Albion during the Premier League match between Arsenal and West Bromwich Albion at Emirates Stadium on September 25, 2017 in London, England. (Photo by Mike Hewitt/Getty Images) Getty Images Arsenal vs West Brom player ratings Allan Nyom - 5 BURTON-UPON-TRENT, ENGLAND - JULY 26: Allan Nyom of West Bromwich Albion looks on during the pre season friendly match between Burton Albion and West Bromwich Albion at Pirelli Stadium on July 26, 2017 in Burton-upon-Trent, England. (Photo by Nathan Stirk/Getty Images) Getty Images Arsenal vs West Brom player ratings Jake Livermore - 6 West Bromwich Albion's English midfielder Jake Livermore (L) vies with Arsenal's Chilean striker Alexis Sanchez during the English Premier League football match between Arsenal and West Bromwich Albion at the Emirates Stadium in London on September 25, 2017. / AFP PHOTO / Ian KINGTON / RESTRICTED TO EDITORIAL USE. No use with unauthorized audio, video, data, fixture lists, club/league logos or 'live' services. Online in-match use limited to 75 images, no video emulation. No use in betting, games or single club/league/player publications. / (Photo credit should read IAN KINGTON/AFP/Getty Images) AFP/Getty Images Arsenal vs West Brom player ratings Grzegorz Krychowiak - 7 LONDON, ENGLAND - SEPTEMBER 25: Grzegorz Krychowiak of West Bromwich Albion is chased by Laurent Koscielny of Arsenal during the Premier League match between Arsenal and West Bromwich Albion at Emirates Stadium on September 25, 2017 in London, England. (Photo by Michael Steele/Getty Images) Getty Images Arsenal vs West Brom player ratings Gareth Barry - 6 LONDON, ENGLAND - SEPTEMBER 25: Gareth Barry of West Bromwich Albion is faced by Mohamed Elneny of Arsenal during the Premier League match between Arsenal and West Bromwich Albion at Emirates Stadium on September 25, 2017 in London, England. Gareth Barry is making a record 633rd Premier League appearance. (Photo by Michael Steele/Getty Images) Getty Images Arsenal vs West Brom player ratings Kieran Gibbs - 7 LONDON, ENGLAND - SEPTEMBER 25: Kieran Gibbs of West Bromwich Albion beats Hector Bellerin of Arsenal during the Premier League match between Arsenal and West Bromwich Albion at Emirates Stadium on September 25, 2017 in London, England. (Photo by Michael Steele/Getty Images) Getty Images Arsenal vs West Brom player ratings Jay Rodriguez - 6 LONDON, ENGLAND - SEPTEMBER 25: Jay Rodriguez of West Bromwich Albion shoots past Petr Cech of Arsenal during the Premier League match between Arsenal and West Bromwich Albion at Emirates Stadium on September 25, 2017 in London, England. (Photo by Michael Steele/Getty Images) Getty Images Arsenal vs West Brom player ratings Hal Robson-Kanu - 6 LONDON, ENGLAND - SEPTEMBER 25: Hal Robson-Kanu of West Bromwich Albion and Shkodran Mustafi of Arsenal battle for the ball during the Premier League match between Arsenal and West Bromwich Albion at Emirates Stadium on September 25, 2017 in London, England. (Photo by Michael Steele/Getty Images) Getty Images

Arsenal went into the clash having lost their previous two outings at Everton and Manchester City, results which saw them slip further away from in-form Chelsea, and Koscielny admitted the players needed the morale boost of returning to winning ways.

"We lost the first game of the season (against Liverpool) and we came back with a good run and a lot of wins," he added.

"We lost two games in a week last week and it was very important to come back with a win. The confidence will come back when you win and the most important thing now is to continue to win games in a row so that hopefully we can get into the first position."

Olivier Giroud heads home Arsenal's winning goal after out-muscling Gareth McAuley (Getty)

The Baggies have now lost three of their last four games, albeit to Chelsea, Manchester United and Arsenal.

Foster was in superb form as he kept out efforts from Sanchez, Iwobi and Giroud before being beaten by the forward's header.

And the England-capped goalkeeper conceded it was a bitter pill to swallow, especially after such a strong team performance.

"It's tough to take," Foster told West Brom's website.

"They were the better team and had a lot more chances and possession than us. Saying that, to lose any game in the last three or four minutes is always hard to take.

Foster made numerous stops until Giroud's header beat him (Getty)

"Giroud is a good, strong player and it was a good header so fair play to him. The result reads 1-0 and we have to take it and get on with it.

"You get to the last five minutes of any game and you'd like to think that you could shut it out. When you come to a place like the Emirates and it's still 0-0 after 85 minutes you'd be really happy.

"In the end though we have lost the game and that's really disappointing. We showed good spirit and discipline to keep them out before then."