Stewart is still rehabilitating from an injury he suffered in January and is not expected to be back in action before September.
However, Southampton are pushing hard to sign the striker from Sunderland and have already tabled two bids for him.
Sunderland’s £10m asking price is yet to be breached by the Saints, but they remain determined to try and sign him before the window closes on Friday night.
However, Mowbray is insistent that unless a new striker is bought in the coming hours, Stewart will not be allowed to leave the Stadium of Light.
The Sunderland manager said in a press conference when asked about Stewart’s situation: “I’d suggest that he can’t be sold unless someone comes in.
“Truthfully, sitting here, I don’t know [what will happen with that one].”
The forward is prepared to make the move but Mowbray is reluctant to sell him without securing a replacement.
Southampton are pushing to sign him despite his injury status and are hoping to work out a deal in the next 24 hours.
