Manchester United have been warned not to underestimate Europa League semi-finalists Celta Vigo

Terry Gibson expects Manchester United to progress to the Europa League final but has warned semi-final opponents Celta Vigo must not be underestimated.

United travel to Vigo on Thursday for the first leg of their European semi-final - their first in six years - looking for a positive result ahead of next week's return at Old Trafford.

Having been paired with Celta over less desirable opponents Lyon and Ajax, there is an air of confidence that United will reach the final in Stockholm on May 24.

But, while acknowledging the cause for optimism, Sky Sports' Spanish football expert Gibson stressed a degree of caution for his former club.

"Having drawn a mid-table Spanish team, optimism from United fans is understandable," Gibson told the La Liga weekly podcast.

"The way the legs fall - the first at the Balaidos and the second at Old Trafford - suits United too.

"Celta may take United by surprise though, because they are a decent team. People are expecting a comfortable Manchester United victory which might happen, but I wouldn't go into the game as a United player, fan or manager thinking this is going to be easy.

"Celta knocked a Real Madrid side at near full strength out of the Copa de Rey this year and they've beaten Barcelona as well.

Former Liverpool striker Iago Aspas has scored 24 goals for Celta this season

"People target Iago Aspas as a weak link because he didn't do it in England during his time at Liverpool, where he barely played, but that argument doesn't stand up.

"I would expect United to go through but it wouldn't be a massive surprise if Celta succeeded over two legs.

"Let's not forget, any team that's in the last four of a European competition deserve to be there."

Three consecutive La Liga defeats to Real Betis, Sevilla and Athletic Bilbao on Sunday have only strengthened support for United to overcome Celta.

However, having rested key players in preparation for the first major European semi-final in the club's history against a United side who played nine times in April, Gibson believes the tie is anything but a foregone conclusion.

Asked whether Celta rotation policy was justified, Gibson added: "I understand where Celta are coming from but I question the wisdom in that. They have got no momentum or consistency in their form.

Celta overcame Genk in the last eight to reach their first major European semi-final

"They are playing a B team in league games and purely fulfilling fixtures. But they are in a historic position for that football club, in the last four of a European competition to be playing Manchester United.

"I get why manager Eduardo Berizzo is protecting his starting XI. I would imagine there is work going on behind the scenes to maintain the fitness levels of the starting XI.

"I would imagine they are also working on tactical systems to use against Manchester United as opposed to none going into the system used in the home loss to Bilbao or the defeat at Sevilla.

"Celta haven't just rested players for the United tie, they've been doing this throughout the competition because it's such a big thing for Celta to be playing European football, it doesn't happen often and they certainly haven't been to the semi-finals.

"It's going to be interesting to see this fresh, vibrant Celta team against United who, as Jose Mourinho has said on several occasions, have been playing a lot of football."