The Portuguese winger picked up an injury just before half-time, when United were leading 2-0 and pressing for what might have been a killer third goal. He played no further part in the match and could only watch as the visitors rallied to force a replay.
Sir Alex told MUTV: "Nani got a little hamstring injury. I don't think it's too serious but nonetheless he had to come off."
It was the second time in a week, of course, that Nani had to leave the field prematurely when the Reds were winning. And while there was no numerical disadvantage on Sunday, with Antonio Valencia coming on as a substitute, we'll never know if Nani could have affected the outcome after his recent return to form.
"That was a blow to us because Nani was playing very well at the time," admitted Sir Alex.
Nani himself had told Sunday's edition of United Review, the official match programme, that his "confidence is high." He added: "This is my time and I think I can be important for the team in these big games. I feel I’m in good form and I’ve nearly arrived at my peak. I think over the next few games I can surprise a few people."