To make matters worse for the London club, Vunipola exited the game with a knee complaint while Farrell sustained an ankle injury.
A Premiership final win against Saints would be the perfect way to cap their years after victory in Australia with the Lions last summer - but McCall is concerned that neither may recover in time.
"We have some concerns over Mako and potentially Owen," McCall told ESPN. "Owen has a small ankle injury which he picked up in the warm-up after he tripped over a wire, which is quite disappointing. We'll see how he is. Mako is the big concern for us."