Sir Alex has confirmed he may make a number of changes to his United line-up for Sunday's FA Cup semi-final against Everton at Wembley.
The boss says that Wednesday's Champions League win in Porto, and its high-speed nature against tough opponents, took a lot out of his players.
“It was gruelling and there was never any period where it was comfortable. It was always played at a high tempo and credit to Porto they kept going," he said. "The pressing of the ball from both teams was the best I’ve seen in Europe for a long time.
“There is a subsequent loss of energy in some parts of my team; I need to recognise that and make some changes on Sunday because it’s a big game.�
Darren Fletcher, who missed the Reds' trip to Portugal, should return but Sir Alex faces a decision on the match-readiness of Wayne Rooney and Michael Carrick. “There aren’t any serious injuries, but we’ve got quite a few bumps and bruises. Rooney got a knock, and Carrick got an ankle knock, but nothing serious. The good news is Fletcher has trained in the last few days and he should be OK for Sunday."
Another boost is the return to full training of Wes Brown. He could taste action on Monday night when the Reserves play Preston North End in the Lancashire Senior Cup semi-final.
"Wes started training with the first team today, which is great news for us. I’m contemplating playing him in the Reserves on Monday. It’s maybe a bit early, but given the time of the season and the importance of the games coming up we may rush him back a little bit. It’s a thought for Monday. Other than that everybody is fine.
"It’d be great to have him available, it would give us options with Gary Neville and young Rafael back. Rafa’s looking fantastic in training actually. That would give us support in the back four positions."
The resumption of Rio Ferdinand and Nemanja Vidic's key alliance at the heart of United's defence was also a major bonus for the Reds. "When you see the back four coming together, with Rio back alongside Vidic and a bit of freshness in the team, it made a hell of a difference to our performance. That was the foundation of the win, I think.
"I actually thought the watershed was last weekend against Sunderland. We defended well and got back to something like our best. We need to be at our best now, and that showed on Wednesday – we were on top form. We take encouragement from it.
"Rio’s experience and composure under pressure at this time of year really helps," he added. "But, more than that, I think it’s the consistency of the partnership with Vidic. I keep going back to it, but Steve Bruce and Gary Pallister played for seven or eight years together and hardly missed a game. Through that they got consistency, understanding and a knowledge of each other’s capabilities.
"That's the same with Rio and Vida. But it’s also about presence within the team; Ferdinand and Vidic’s presence on Wednesday was outstanding."