The former Dortmund captain managed just 45 minutes of Tuesday's 2-0 win over Arsenal in the UEFA Champions League before being replaced by Matthias Ginter.
Goals from Ciro Immobile and Pierre-Emerick Aubameyang did the damage as Dortmund dominated their Premier League opponents at Signal Iduna Park.
But the win came at a cost, with Kehl joining the likes of Marco Reus, Mats Hummels and Ilkay Gundogan in the treatment room.
Though the 34-year-old's injury is not thought to be serious, Kehl put his involvement in Saturday's trip to Mainz in doubt.
"I had groin problems and it made no sense to continue," he said.
"I don't know if I will be fit in time for Saturday."
Immobile opened the scoring late in a first half which Dortmund had completely overrun, bursting through Arsenal's midfield and defence before slotting past Wojciech Szczesny.
And Kehl believes a first goal in Dortmund colours will do the Italy forward the world of good.
Kehl added: "He made a big step forward. The fans cheered him wildly. This will do him good."
