Boss Mauricio Pochettino hopes an injury suffered by striker Christopher Nkunku is not a "big issue" as Chelsea drew 1-1 with Borussia Dortmund to end their pre-season US tour undefeated.
Mason Burstow scored a 90th-minute equaliser for Chelsea after Marius Wolf gave Dortmund the lead in Chicago.
£52m signing Nkunku, 25, went off in the 22nd minute after appearing to injure his knee when he was tackled.
The pitch at Chicago's Soldier Field was in poor condition.
Areas were patchy and worn after the stadium, home to the NFL's Chicago Bears, was used for an Ed Sheeran concert attended by 73,000 people last weekend.