The Championship side signed the Ukrainian forward from Zorya Lugansk on deadline day in the recently concluded summer transfer window.
Sunderland were expected to sort out all paperwork and a work permit for the striker to be available to play for the club immediately after the international break.
However, Mowbray conceded that as things stand Sunderland are still waiting for the work permit to get processed and Rusyn is not cleared to play for them yet.
He did provide a glimpse of hope that by the time the game kicks off that could change and the Ukrainian could be available for the clash against Queens Park Rangers on Saturday.
Mowbray said in a press conference: “We are waiting on visas and what have you.
“As I sit here now, he’s not available [against QPR], but that might change between now and then.”
Rusyn scored once at the start of the Ukrainian top flight before he moved to Sunderland on deadline day.
