"Wes has a hamstring strain and it's going to keep him out for a couple of weeks," Sir Alex confirmed on MUTV.
"Vidic suffered a calf strain tonight and that's a worrry for us ahead of Saturday. We just have to get on with it.
"Young De Laet has shown some promise and he did very well when he moved to centre-back. He is a natural centre-half and he'll probably be there on Saturday."
De Laet was making only his second league appearance for United, following his final-day debut at Hull City in May. He completed 90 minutes, in a back four that finished with Darren Fletcher at right-back, Michael Carrick in the centre and Patrice Evra on the left.