The 20-year-old joined Sunderland from Arsenal in a £2million move in 2007.
“We are a little disappointed with Anthony Stokes because he has great ability, but sometimes he is not fully focused,” Sunderland boss Ricky Sbragia said.
“He is great on the technical side of things, but it is just getting the best out of him.
“Early doors he did quite well at Sheffield United, but then he faded and I was disappointed.
“If we send a player down to that level, you are expecting him to do well.”