On 31 May 2007, it was announced that Hargreaves would join Manchester United on 1 July 2007, after almost a year of negotiations between Bayern Munich and United for a fee of around £17 million.] Hargreaves was finally unveiled as a Manchester United player on 1 July, having signed a four-year contract with the club. He was then revealed to the press on 9 July, along with fellow newcomer Nani. Hargreaves was given the number 4 shirt at Old Trafford.
Hargreaves made his Manchester United debut in a friendly against Peterborough United on 4 August, coming on as a second-half substitute in United’s 3–1 victory. He made his first Premier League appearance in Manchester United’s third game of the season, in the derby match away to Manchester City in a 1–0 defeat. Hargreaves scored his first league goal for Manchester United against Fulham on 1 March, a free kick from just outside the penalty box.He scored his second league goal for the club on 13 April, a free kick winner against Arsenal. In his first season with Manchester United they won the Premier League and Champions League titles. In the Champions League final, Hargreaves played all 120 minutes, starting on the right of midfield, and scored United’s fourth penalty of the shootout as they defeated Chelsea 6–5 on penalties.
The start of Hargreaves’ second season at United was hampered by injuries, specifically a recurring patellar tendinitis problem that restricted him to sporadic appearances in 2008–09. After trips to specialists in both London and Sweden yielded no results, Hargreaves travelled to Colorado, USA, in November 2008 to visit renowned knee surgeon, Richard Steadman.[ Hargreaves underwent surgery on his right knee on 10 November 2008 and received a similar operation on his left knee in January 2009. However, this meant that Hargreaves would miss the remainder of the 2008–09 season, ultimately meaning he would miss out on a second Premier League title as well as League Cup, FIFA Club World Cup and FA Community Shield winner’s medals.[Hargreaves underwent his rehabilitation programme in the United States, and was due to return to training with Manchester United at the Audi Cup in Munich. However, upon assessment, it was decided that he should continue with his rehabilitation in the United States. Hargreaves returned to Manchester United on 23 September 2009, joined by his physiotherapist for a week to aid the transition process and for the club to understand his rehabilitation programme.
In January 2010, it was speculated that Hargreaves was initially slated to return in time for Manchester United’s Champions League first knockout round tie against Milan, despite not being fully match-fit However, on 4 February 2010, Hargreaves suffered a major setback to hopes of playing in the 2010 FIFA World Cup as Sir Alex Ferguson omitted him from the squad for the knockout stage in favour of Belgian defender Ritchie De Laet. It was expected that Hargreaves would make his return from injury in a reserve team match against Manchester City on 11 March,[but on the day of the match, Hargreaves pulled out of the game to continue with his training regime. He eventually made his comeback a week later, playing for 45 minutes of Manchester United’s reserve team match against Burnley on 18 MarchOn 24 April, Hargreaves was named as an unused substitute for the first-team match against Tottenham Hotspur