In the 2009 season, he scored 16 goals in 29 matches. Even with his strong performance, Molde only finished second in the league and the cup. A highlight of the season was the 5–2 win against Brann on 12 July 2009, where Diouf scored a hat-trick in the first nine minutes of the game, and added the fourth goal in the 27th minute.
In May 2009, he received interest from several senior clubs and on 17 July 2009, Molde and Manchester United announced that a transfer had been agreed for the player. Diouf was loaned back to Molde for the remainder of the Norwegian 2009 season, before joining his new teammates in Manchester in January 2010. "We signed a young player yesterday from Norway, who we've been looking at for two years," United manager Alex Ferguson said. "We weren't intending on signing anyone else after last week – we feel we have a full squad – but the situation accelerated to a point where other clubs started to make bids, so we had to decide whether we acted or didn't act. We decided to act and he'll be the last person we sign."
On 23 December 2009, Diouf was granted a work permit making him available to play for Manchester United once the transfer window opened in January. After becoming eligible to play for Manchester United on 7 January 2010, Diouf was registered with the number 32 shirt worn previously by Carlos Tevez, and made his debut in a league match away to Birmingham City two days later; with the scores level at 1–1, Diouf came off the bench for Paul Scholes in the 81st minute, but he was unable to help break the deadlock and the match finished as a draw.
On 12 August 2009, Diouf won his first cap for Senegal in a 2–1 friendly win over Congo DR in Blois, France. He scored his first goal for Senegal in a 1–0 victory over Cape Verde on 11 August 2010.