Arne Slot confirmed Alexander Isak was replaced at half time of Liverpool's 5-1 win at Eintracht Frankfurt due to a groin injury.
The striker started alongside Hugo Ekitike in Germany, but his night was ended at the break as he was replaced by Federico Chiesa.
Isak's form has come under scrutiny since his £125m move from Newcastle this summer, having only scored in the Carabao Cup for his new side.
After the Champions League encounter - which brought an end to a four-match losing run - Reds boss Slot confirmed Isak came off because of injury.
He told TNT Sports: "Alexander had to go off at half-time because he felt his groin a little bit. That's a pity.
"I've said many times it is such a difficult balance to find with a player that missed three months.
"You go slowly with him and p