Martinez sold former Latics midfielder Valencia to United.
He said: "If you speak with anyone who worked with Antonio here, they will tell you what an extraordinary character he is.
"We all know the quality he has got and now he looks a fresh player.
"I think Antonio looks a real threat. He looks really sharp and he is enjoying his football, at his best level.
"It seems as though he has been a key man in every fixture he has played and we are all very happy on one hand, because Antonio left a massive stamp on the football club.
"But on the other hand we hope he has a bit of a day off against us."