£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.

"The doctors are checking him," explained Pochettino, who felt the tackle which forced Nkunku off "maybe" should have resulted in a penalty.

"He had the chance to score but he feels something in his knee.

"Hopefully it is not a big issue and that he can be quick with the team. We need a few days to assess him and until we get back to London it is difficult to know.

"Always we take some risks because the pitch is not perfect as they are using it for different sports and that is sometimes the risk of the tour.

"I think it was a bit unlucky in the situation but we cannot complain about the pitch and to blame the pitch for why he suffered the injury. For me, it was bad luck."

Chelsea had the best chances of the first half when Nicolas Jackson had a shot saved by Alexander Meyer after Nkunku was taken down in the area.

Levi Colwill should have scored when Thiago Silva flicked the ball across the face of goal from a Ben Chilwell pass, but he shot over the bar from inside the six-yard-box.

Chelsea started the second half well and Jackson fired over after running onto a bouncing ball over the top of the Dortmund defence, before Carney Chukwuemeka had a shot blocked on the line after good work from Raheem Sterling and Jackson.

Dortmund took the lead in the 80th minute when Wolf scored from close range, turning in Samuel Bamba's shot.

Chelsea then drew level when Burstow headed in from inside the six-yard box after Enzo Fernandez headed a Mykhailo Mudryk free kick to the teenage striker.

The Blues begin their Premier League campaign against Liverpool at Stamford Bridge next Sunday, 13 August.