A massive crowd of 73,738 were treated to a highly entertaining 90 minutes between the champions of England and Italy.
Roberto Mancini's Inter won thanks to a brace from David Suazo and a neat finish from Zlatan Ibrahimovic, while Wayne Rooney and an Adriano own goal were United's efforts.
The Italian champions appeared sharper in the first half, but United were clearly on top after the interval and were unfortunate not to at least take the game to a penalty shootout.
Injuries prevented Sir Alex Ferguson from fielding any of his three summer signings, with Anderson, Nani and Owen Hargreaves watching from the stands.
Within three minutes Ronaldo had given them and the rest of the packed crowd a sample of his skills, tormenting Maxwell with his beguiling array of tricks before dragging a shot wide of Cesar's right-hand post.
Three minutes later the Portuguese winger was again at the centre of United's attacking menace, sending a teasing cross towards Eagles at the back post. New Inter signing Christian Chivu, last seen at Old Trafford in Roma's 7-1 mauling last April, just about managed to clear under pressure.
Scares for United were few and far between in the game's opening exchanges, thanks largely to the profligacy of Ibrahimovic, who first completely missed a free header from Figo's corner before cheaply surrendering possession when Inter had three attackers against two United defenders.