Roy Keane has accused Manchester United of cutting corners in the transfer market.

The Reds have taken the first steps in strengthening their side by signing the Spanish midfielder Juan Mata for £37m from Chelsea, but it has become clear that their new manager David Moyes will need to spend big in the summer transfer window if he is to halt a decline that has seen the champions slide to seventh in the Premier league.

Since the Glazers took over the club in May 2005, they have spent over £696m on interest fees, bank charges and debt repayment, and around half that much on players: £382m.

Keane told itv.com: "They've kind of cut corners in terms of transfers in the last few years, they've not gone out and got the big players, and it's just caught up with them this year."

United already face a fight to qualify for the Champions League this season and they will be desperate to get past the Greek side Olympiacos in the final 16 of the competition on 25 February, which is by no means a foregone conclusion.

Keane, however, sees no reason to be overly pessimistic about their future under Moyes. "I think Man United need five or six players. If it happens in the summer then instead of being fearful of it, embrace it – it's exciting. These can be exciting times for Man United. Get them back to the very top where they belong."

United's former captain, now an ITV pundit and assistant to Martin O'Neill at the Republic of Ireland, added: "There's always something to play for. You've always got to win your next football match so if you're telling me in January that your season is over, or when the Champions League starts, that's nonsense.

"You're playing for pride, you're playing for points, and United, if they do go out of the Champions League, still want to get in it next year.

"I think beating Olympiacos will be the priority. On form at this moment in time you have to say the Champions League is a long way off for Manchester United. But lots will change over the next few months. I think United will beat Olympiacos," Keane said.

"They'll have Rooney, Van Persie back after injuries. At this moment in time, would I fancy United for the Champions League? Probably not. But they're still a threat, they're still there. Big team, big players and in a couple of months' time when the big players are back up to speed with their fitness United will be a dangerous team.

"Watching United more closely perhaps than other people, you still think the Van Persies and the Rooneys if they're at it, will be a threat to the competition. I still think United can be dangerous if they get their act together over the next month or two."

Keane also felt that Moyes needed to be given breathing space to make his mark on the side. "I think the man deserves plenty of time. I think people have been afraid to use the word that there might be 'rebuilding' going on but that's exactly the word I would use."

Roy Keane will be appearing as a pundit on ITV's live Champions League coverage