Adam Reach and Daniel Pudil sustained knocks before and during the Owls' poor 2-0 defeat to Brentford today.
Reach, who registered in their Carabao Cup triumph at Sunderland last Thursday, picked up a knee injury during the warm-up, forcing manager Jos Luhukay into making a late change to his starting eleven. Marco Matias was drafted in and spurned a glorious chance in the first half to cancel out Neal Maupay's 20th minute penalty.
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On Reach, the Owls' reigning player of the year, Luhukay said: "It was not possible for Adam to play. He had too much problem and pain in his knee.
"Hopefully it is not serious. Adam is one of our important players. We saw that last Thursday.
"We must wait until tomorrow (Monday ) to see what the doctor maybe does with him."
As for Pudil, he picked up an unspecified knock and was replaced with six minutes remaining by Matt Penney.
"Daniel Pudil went off the field and I don't know if he will be available for Wednesday (the Millwall game)," admitted Luhukay.
Meanwhile, the Owls are set to learn the full extent of Joost van Aken's ankle problem tomorrow. Luhukay is also sweating on the fitness of Morgan Fox (back) and Lucas Joao (groin) as Wednesday prepare to entertain Millwall.
Luhukay said: "The injuries are no excuse. We must play with a team who can play and give 100 per cent. We must see a team who play on Wednesday give a better performance."