The Senegalese striker was signed from Molde in 2009 for £3m but with limited opportunities at Old Trafford, went on loan to Blackburn before completing a move Hannover in January having made just 9 appearances for the club.
At Hannover with playing time more forthcoming, the 24 year old has managed to reignite his career with 15 goals in 24 appearances.
Speaking to German paper Bild, Diouf said:
“Manchester United is a great club and I have enjoyed being a player of the club.
“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.”