Celtic coaching staff say a lack of conditioning is the reason why Daniel Arzani was left out of the Hoops' Europa League squad.
Arzani has joined Celtic on a two-year loan from Manchester City.
The Australian winger has yet to make his first-team debut for the Scottish side.
Celtic had to submit its A and B squad lists to UEFA, a total of 43 players, and the 19-year-old missed the cut.
"He still has a way to go to get up to speed but he has a two-year loan and we are going to support his development," assistant coach Chris Davies said.
"His main priority is to build up his condition, he’s had half a development team game. The style we have is fast-paced and intense so players have to be at that level."
Arzani has the chance to be added to the Glasgow outfit's Europa League squad if they make it to knock-out phase.