The Black Cats signed the Ukrainian striker on deadline day of the last transfer window but he is yet to make an appearance for Sunderland.
It has been close to three weeks since the signing was confirmed and Sunderland are still waiting for the work permit for the forward to go through.
He was not available for Sunderland’s 3-1 win over QPR at the weekend but Mowbray insisted that the club were certain of getting the visa sorted this morning.
However, to the utter surprise of Sunderland, Rusyn’s work permit has not arrived yet and he will not be with the squad to face Blackburn Rovers at Ewood Park on Wednesday night.
Mowbray said in a press conference: “Nazariy Rusyn still hasn’t got his visa so he won’t travel.
“We’re surprised it’s not through.
“We had every expectation it would be here this morning, but when we came back in from training it hadn’t arrived.”
It remains to be seen whether the new signing will get the work permit to be available against Cardiff City on Sunday.
