The Irish international played in the first half, during which Dutch ace Arjen Robben scored (12th) for the English champions. Chelsea coach Jose Mourinho then made eight changes at the interval.
Duff and company play DC United at RFK Stadium in Washington, DC, this Thursday (kick-off 8 p.m.) before facing Milan again at Giants Stadium in New Jersey on Sunday. That match starts at 3 p.m.
Meanwhile, former Republic of Ireland coach Mick McCarthy and his English Premier League (EPL) promotion side Sunderland completed a three-game jaunt to North America with a goalless tie against Portland Timbers of the United Soccer League (USL) in Portland Oregon on Sunday.
McCarthy’s men had earlier bounced back from a 3-0 thrashing by the Vancouver Whitecaps in their opener to edge the Seattle Sounders 1-0 on a Liam Lawrence goal (81st) last week.