Westwood is second choice behind Simon Mignolet at the Stadium of Light and that has led to him losing his shirt for the Republic.
According to their assistant manager, Marco Tardelli, Westwood tried to get a move away from the Stadium of light during the January window.
"He tried (to leave), but O'Neill (Martin) decided it was better that he stayed at Sunderland, and Giovanni (Trapattoni) can't do anything about that," said Tardelli.
"For me, it would have been better if he had moved."
Republic boss Giovanni Trapattoni has suggested he might struggle to get his place back, with Millwall's David Forde getting the nod instead, while he continues to sit on the Sunderland bench.
"The goalkeeper is a particularly delicate position," Trapattoni said.
"I don't worry about an outfield player if he is not playing for his club but a goalkeeper is alone on the pitch, and if he hasn't been playing it's more difficult psychologically.
"I hope Westwood can play in the future but now the manager chooses (Simon) Mignolet.
"I told him you can change club and you can play."