United manager Alex Ferguson left summer signings Nani, Anderson and Owen Hargreaves on the sidelines as his team laboured.
Inter recorded the first victory of their pre-season tour to England with two goals from striker David Suazo and another from Swede Zlatan Ibrahimovic - all in the first half.
Wayne Rooney, by far United's most dangerous player, had opened the scoring in the first half with Adriano heading into his own net to gift the Premier League champions a second.
England defender Rio Ferdinand wasted a chance to equalise by shooting over the bar from close range with nine minutes left.
United face Chelsea in the Community Shield at Wembley on Sunday, the curtain-raiser to the new season.
