The 22-year-old has impressed United boss Sir Alex Ferguson in training and has been granted international clearance.
United have handed Diouf the No32 kit, previously held by Carlos Tevez.
Ferguson said: “He has adapted very well to our training regime and has looked very impressive.
“He is quick, very good in the air and a good striker of the ball.
“Overall, he is a really good centre forward and he will play a part this season.”
