David Moyes has admitted it is frustrating that he will be without Juan Mata in the UEFA Champions League but he still believes Manchester United can challenge for European glory.
Mata is cup-tied for the Champions League after signing from Chelsea in January, but Moyes has defended the decision to bring in the Spaniard when they did for a club-record fee.
He accepts that not many major signings are made in the winter, but insists it was too good an opportunity to miss.
"We wanted to get Mata when we could and that was the January transfer window," said the United boss.
"Yes, it's a frustration we can't use him in the Champions League and that's why you hear people saying January isn't a great time to buy.
"Most of the clubs are trying to buy players who aren't Champions League cup-tied.
"But the better ones tend to be - like Juan. We are trying to buy that type of talent, so as I say, we wanted to get him early when we could."
United are struggling to qualify for next season's Champions League by virtue of finishing in the top four of the Premier League, but Moyes believes they could still get there by winning this season's competition.
The Red Devils safely negotiated their group before Christmas and have been drawn to face Olympiakos in the last 16, with the first leg taking place in Greece on Tuesday.
Moyes said: "We performed well in the group games, when we finished top. That was the first aim.
"And in any cup competition you have a chance - the best team doesn't always get to the final. Look at the FA Cup Final last season, when Wigan beat Manchester City.
"We now have to try to beat Olympiakos over two legs and that will be no mean feat, because they are undoubtedly the best team in Greece and a lot of their players will be going to the World Cup as part of the Greek squad.
"Everyone in this competition wants to win it and we will try to do that.
"Teams don't want to play Manchester United, because they know what this club stands for.
"And the only thing domestically that I am concentrating on is trying to finish fourth and getting that Champions League spot."