Manchester United are waiting to discover the seriousness of an injury sustained by Shinji Kagawa after the midfielder twisted his knee against Braga.
The Japan international suffered the damage during the first half of Tuesday night's thrilling 3-2 comeback win in the UEFA Champions League.
The result virtually guaranteed the Red Devils a place in the competition's knockout phase with three games remaining.
But it may come at a price, with Kagawa due for tests on Wednesday to discover the extent of the problem.
"Kagawa twisted his knee," said Ferguson. "He struggled on for 20 minutes but we will assess in in the morning because we don't know the extent of it at the moment."
With goalkeeper Anders Lindegaard also ruled out for a week after suffering a thumb injury in training, Ferguson is likely to be down on numbers for Sunday's crucial Premier League trip to leaders Chelsea.
Of more concern to the United boss is his team's inability to keep the opposition out after they found themselves behind for the eighth time in 12 games this season against Braga.
"I can't get to the bottom of it, I am afraid," he said. "If you analyse the goals we are losing, they have come from throw-ins, crosses, cut-backs - all sorts. And they are finding players free in the box.
"It is difficult to put your finger on it, and it is a concern because it is making it difficult for us."