The Belgian international broke his nose in a clash with Emile Heskey on Saturday in the 2-2 draw with Aston Villa and he faces a spell on the sidelines.
Summer signing Keiren Westwood was outstanding when he came on in Mignolet's place, but after him there is little cover.
Therefore, Bruce may ask if he can bring someone in as Craig Gordon is still not ready to return from a knee operation and fourth-choice Trevor Carson has a back injury.
"Craig (Gordon) is nowhere near (returning) and Trevor Carson is at Bury on loan and hurt his back in the warm-up," Bruce told the Journal.
"With Simon we will have to see how bad he is, and if it is only just a broken nose he will be able to have an operation but I would not think he will be available for Manchester.
"We may have to look for special dispensation and maybe get a loan somehow. We will see how it is."