United thought they had agreed an £8m deal for a player who cost Fulham £10,000 from non-league Maidstone 18 months ago and has only started two games in the Barclays Premier League.
But Arsenal's attempt to hijack the deal meant United, in desperate need of defensive reinforcements, had to pay an extra £2m for the England Under 21 man.
United way: Chris Smalling will join champions Manchester United at the end of the season despite late interest from Arsenal
United boss Sir Alex Ferguson intervened on Monday with a personal phone call to Smalling, who travelled to Manchester for his medical on Wednesday, hours after playing in Fulham's 2-0 defeat at Tottenham.
United's decision to announce the signing before the game at Spurs also added pressure on the player. It was a subtle reminder from Ferguson that he thought they already had an agreement.
Fulham chiefs heap praise on United bound 'diamond' defender Smalling Man United 3 Man City 1: Wayne Rooney finds the killer blow to end the gallant City fightMANCHESTER UNITED FC