Sir Alex Ferguson conceded Norwich City’s spirited resolve forced him into a tactical switch which helped the champions-elect sweep away Chris Hughton’s men at Old Trafford.

The Manchester United boss was sufficiently concerned at City’s greater attacking urgency around the hour mark to deploy Japanese hat-trick hero Shinji Kagawa in a more central role to devastating effect.

Kagawa’s scuffed opener in first half stoppage time had punctured Norwich’s resistance before a late brace and Wayne Rooney’s fearsome strike from 25 yards sealed a comfortable Premier League victory.

“Midway through the second half, I didn’t think it would end up 4-0 as I think we were a bit ragged for that period,” Ferguson told MUTV. “But, when we brought Shinji into central midfield, it made a difference to us and he ended up with a hat-trick. It’s brilliant for him. The lad is a good finisher and his second goal was so composed and an intelligent finish while his third was absolutely brilliantly-taken.

“It’s been a great day for him. He got that injury around October-November and was out quite a while. It knocked him back a bit but he’s gradually coming back and I think you’ll see a really good player next year.”

Ferguson was less impressed with what he felt was Norwich’s physical approach. Referee Neil Swarbrick booked a quartet of City players, and Ferguson felt Robert Snodgrass was fortunate to escape further sanction after tangling with Patrice Evra.

“I actually thought the boy could have been sent off in the first half. But probably the referee’s done the right thing – no controversy,” he said. “Danny Welbeck had a real whack near the end. It’s very unusual to get that kind of game at Old Trafford because the pitch is so big.”

Ferguson admitted the result was all that mattered after opening up a 15-point gap over their neighbours ahead of Manchester City’s trip to Aston Villa tonight.

“The name of the game at the minute is to win our matches and keep that gap between us and our rivals,” he said. “It is a cushion, no doubt. I expect City to win at (Aston) Villa because they have a young team, they are struggling hard at the bottom of the league but if we win our games nothing else matters.

“If we win our next game, then the next, we don’t need to look back. There is a great atmosphere in the dressing room. That is helping us. No matter what comes up, they are relishing it.”

The United boss opted to delay Ryan Giggs’ bid to play his 1,000th competitive game of his career which is now likely to come in the grand setting of Tuesday’s Champions League return against Real Madrid.

“He will be involved and no doubt about it, it will be a great moment for him,” said Ferguson on the Welsh icon. “He is a great example to young players in the game that you can play 1,000 games.

“Team selection is always difficult because the squad is playing really well now. Every one of them is contributing. We have a great spirit and when you are leaving players out you know you are making tough choices because you could pick three or four teams.”

Robin van Persie and Rooney were paired in a potent front two against the Canaries, with the England man notching his first club goal in a month with a thumping finish.

“It’ll do him good,” said Ferguson. “It was good to get Wayne the full game. He scored a goal, which is always important for a striker. Confidence is part of it for Wayne. He’s missed two or three games recently but he worked hard and he scored a goal which is all we need. All in all, we showed good spirit, we were determined and worked really hard.”