The Black Cats could not build on their 4-0 victory over Stoke last weekend and Bruce highlighted the side's poor passing game as a key factor in their loss.
"We didn't pass the ball as well as what we needed to. We gave the ball away so many times so cheaply.
"Today we had too many people who gave the ball away cheaply. The level of performance from a lot of the players was below par and that makes it difficult at this level."
Sunderland currently sit 14th in the Premier League table, with five points from six games.