Diouf, 24, managed only nine appearances for the Red Devils and despite a successful loan spell at Blackburn, could not break into United's first-team plans.
The Senegalese forward eventually secured a move to the Bundesliga with Hannover signing him for an undisclosed fee and ending two and a half years of sitting on the sidelines.
Since the move, Diouf has proved to be a great success, managing to play on a more regular basis and netting 15 times in 24 matches.
"Manchester United is a great club and I have enjoyed being a player of the club," Diouf told Bild.
"But when at such a big club like United you need someone that fully believes in your skills. It's useless just to be one more good player.
"It is a fact I wanted to move on. I did not want to have another loan spell at another club."