The fullback was outstanding for victory at Manchester City on Saturday.
But Fletcher told TV2: "He loves to defend himself. It's refreshing to see, but he has to get better with the ball. Hopefully he is working on receiving and passing in training. I don't see for myself that he's going to be an attacking fullback who storms ahead.
“I was talking to some Manchester United players, and they were saying that no player has managed to pass Wan-Bissaka in training this season.
“It's as if he refuses to let them pass. He has to get better technically, but he is a fantastic fullback. When he gets better technically, he's going to take his game to a new level. He is a fantastic defensive player and contributor for Manchester United."