José Mourinho believes Anthony Martial is struggling to adjust to the intensity he demands from Manchester United and has warned that, if the Frenchman does not grab his chance when selected, others will “take the bait”.

Martial enjoyed a standout opening season at United when he was arguably the club’s best player following a £36m move from Monaco in September 2015. The forward scored 17 times in 45 appearances and was an automatic choice under Louis van Gaal, initially at centre-forward before moving to the left after Marcus Rashford’s emergence.

This season, though, he has struggled for form and is not always a first-choice. Van Gaal’s side were notable for a sluggish tempo and Mourinho believes the change of pace has hindered Martial. Asked whether the 20-year-old was suffering second-season syndrome, the manager said: “First of all the teams are very different. The way the team played last season – I’m not saying better or worse, just different – was probably more adaptive to Anthony. He was probably more comfortable playing that way, at that intensity, at that [rate] of ball possession and ball circulation.

“Probably he also needs some time but in our squad, especially in these positions, we have Mkhitaryan, Lingard, Depay, Mata, Rashford. These are probably the positions where we have more options. You have one opportunity, you have two, you have three – if you don’t bite, then somebody comes and takes the bait.”

Martial was not in the 18 for Thursday’s 4-0 defeat of Feyenoord, a result that means United need only a draw at Zorya in the final Europa League group match to reach the knockout stage. Instead Henrikh Mkhitaryan started for the second time.

“He was OK for this game,” Mourinho said of Martial. “It was just an option [to leave him out]. He played last Saturday against Arsenal. I cannot play with 12 players. I cannot have on the bench 19, 20. You wanted to see Mkhitaryan and I wanted to too. I cannot play all of them at the same time. It was just an option.

“The players have to win their positions. They are friends, they are colleagues who work together every day, but in the end they compete between themselves to have more chance to play. Anthony played against Arsenal. Thursday was a moment to give a chance for Mkhitaryan to start.”

Mourinho warned the Premier League leaders, Chelsea, that their nine-point advantage over United, who are sixth, can be bridged.

He said: “When I won the last title [at Chelsea] 18 months ago I had 10 points’ advantage and then in one month I had the same points as Man City. We lost 10 points in one month. I think it was the end of December or the beginning of January and we’d lost 10 points. Then we recovered and won that title. You can recover points. Others can lose points.

“During the season there are moments when everything goes against you and moments where everything goes in your favour. We know that the [challenge] is not over but the reality is there is a distance. And there are many quality teams. Let’s go match by match and see what happens.”

On Sunday West Ham United are the visitors. “We play another difficult game on Sunday and see if we can get the three points,” said Mourinho, who suggested his side are capable of repeating the emphatic nature of the Feyenoord victory in the league. “One day it can happen,” he said. “Big results are not very usual in the Premier League. We lost 4-0 [at Chelsea] and sometimes somebody loses 5-1 or whatever but it’s not very usual. There is a great balance between the teams. Every game is very difficult. Rather than winning one game 5-0 we need to win four or five matches. That’s what we need. Let’s try to get points.”

After Michael Carrick played all of the draw with Arsenal and the win over Feyenoord Mourinho would not be drawn on whether the 35-year-old could play a third match in eight days by facing West Ham.