Champions League: Alex Ferguson acknowledged for the first time yesterday that he had made damaging errors this season, predominantly by "picking the wrong teams". It was a rare public admission which will enhance Arsenal's belief that they can extend their unbeaten Premiership run to 50 games at Old Trafford on Sunday.

Ferguson was in unusually self-critical mood as he prepared for tonight's tie against Sparta Prague. United should not encounter too many problems here, but their indifferent league form has left them 11 points adrift of Arsenal and the United manager did not attempt to absolve himself of blame.

"Maybe I've made too many changes - that has to be considered," he said. "We're not getting any consistency and that is definitely affecting our performances. At the moment I'm picking the wrong teams for a variety of reasons."

He cited the 0-0 draw at Birmingham when he picked Roy Keane and Cristiano Ronaldo, who had both just returned from a week on international duty. "I gambled on Roy and it turned out he wasn't 100 per cent. I also thought Ronaldo would be fresh enough to play but he looked tired and the first thing he did on the team coach going home was fall asleep. So I knew I'd made a mistake there."

Ferguson, usually hostile to even the mildest criticism, admitted he had made another significant error in not allowing all the players involved in Euro 2004 to have a longer summer break. He meant in particular the decision to fly out Paul Scholes and Mikael Silvestre to the final stages of the tour of the US after criticism in the American media that too many stars were missing. The poor form of both has been a feature of United's start to the season, and Scholes has missed a month because of a groin problem, an injury which Ferguson said was related directly to being "called back too quickly for pre-season".

Silvestre was dropped on Saturday after a series of unconvincing displays, although he will return tonight because Rio Ferdinand has stayed at home to attend his grandmother's funeral. Keane is also absent with a virus, and Quinton Fortune has suffered a recurrence of a calf injury and may miss three weeks. Gabriel Heinze limped out of training yesterday and his fitness will be assessed this morning.

Ferguson is nevertheless entitled to believe his side do not face a difficult night at the Sparta Stadium. Their hosts have lost their opening two games in Group D, against Fenerbahce and Lyon, and Karel Poborsky's presence for the Czech champions should not trouble any United followers who remember his dismal stint at Old Trafford.

Sunday's game is a different matter entirely, and Ferguson was his usual bullish self when the subject turned to the Premiership's early pacesetters. "I accept that Arsenal, last year and at the moment, are ahead of us, but we know we're capable of winning and I think they know that too.

"A win against Arsenal could turn the whole season on its head. It stops their unbeaten run, for starters. Right now they're like a rolling stone that gathers no moss. Somewhere down the line somebody has got to stop them, and you never know what psychological impact that could have.

"I wouldn't say it's win or bust for us, but it's certainly very, very important."

The United manager will not be too distressed to learn that Arsenal may be without Patrick Vieira on Sunday, after he suffered a sprained ankle when fouled by Aston Villa's Jlloyd Samuel during their 3-1 win at Highbury on Saturday. Ferguson also defended his accusation of Arsenal "getting away with murder" in escaping heavier sanctions for the clashes at the end of last season's corresponding fixture.

"My comments were accurate. It was a response to comments supposedly made by Arsene Wenger that we were a 'spent force'. There is a bit of kidology between the two of us and I thought it was appropriate to remind them of what happened last year.

"I note that Arsene has said himself they can't afford anything like that to happen again, and he's quite right."

SPARTA PRAGUE (4-4-2, probable): Blazek; Pergl, Homola, Kovaz, Cech; Poborsky, Sivok, Zelenka, Urbanek; Pacanda, Jun.

MANCHESTER UNITED (4-4-2): Carroll; G Neville, Brown, Silvestre, Heinze; Miller, Djemba-Djemba, Scholes, Giggs; Saha, Van Nistelrooy.

Referee: M De Santis (Italy).