The 25-year-old had to be substituted in the 22nd minute at Soldier Field in Chicago, where he was replaced by Mykhailo Mudryk.
Playing on what was a shocking pitch, the versatile Blues star hit the deck after being challenged by Mats Hummels in the box.
He managed to continue for over ten minutes, but once again fell to the floor, with club doctors providing the France international treatment.
But he was unable to carry on competing, leading to fears from fans that the summer signing could be set for a spell on the sidelines.
Speaking after the match, Pochettino provided an update on the player's niggle, where he failed to give a time frame on when Nkunku will next be on the pitch.
READ MORE ON CHELSEA
Chelsea end pre-season unbeaten as academy star scores dramatic equaliser
Chelsea blow as Christopher Nkunku suffers suspected knee injury against Borussia Dortmund
Quoted by Chelsea's website, he said: "The doctors are checking him and I hope it is not a big issue.
"He fell in the action [against Hummels] which was maybe a penalty and he feels something in his knee, but we hope it is nothing big.
"We hope he can be back quickly with the team. We need a few days to assess him."
Pochettino then failed to blame the condition of the pitch as a possible factor in Nkunku's knock.
Most read in Football
Hojlund teases Man United announcement, new club in Caicedo race, Gvardiol to Man City
'You sure?' - Ferdinand takes Jennings to task on opinion that West Ham should sack Moyes
Arsenal 1-1 Monaco (5-4) LIVE: Ramsdale proves penalty hero as Gunners retain Emirates Cup
Chelsea transfer news LIVE: Premier League club launches Caicedo bid, Mbappe keen on move
He stated: "We took some risks because the pitch is not perfect because the facilities are used for a different sport.
"We won't blame the pitch for the injury though. For me, it was bad luck."
Nkunku has been one of Chelsea's stars of pre-season since joining from RB Leipzig - scoring in three separate matches against Wrexham, Brighton and Fulham.
But he could now miss the west Londoners' first Premier League game of the new campaign against Liverpool at Stamford Bridge on August 13.