Director of Football Nick Hammond explained to the official website, "It's fair to say that Greg did not make the impact here that both he and ourselves would have liked.
"Sometimes in football, players do move on quite quickly if it's felt that the 'fit' isn't quite right. But this is absolutely no reflection on Greg as a person or a player, and he has been a very good professional during his short time with us.
"From a purely financial perspective, this is an excellent deal for us with a good upside on our initial investment in January.
"Sunderland have pursued the player strongly and I'm sure he will do very well for them. Everyone at Reading FC wishes Greg every success for his future."