Ousmane Dembele returned to full training this week after six weeks out but Saturday's game against Napoli will come too soon for him.
The France forward has not been named in the 22-man squad, although Barça coach Quique Setien says he did think about including him.
"I thought about calling up Dembele, but unfortunately he's only done a few sessions with the group and it would have been too much of a risk," Setien said.
"It was not worth it in the end. What I am sure about is that we will take him to Lisbon to train with us and there we will see if he can play."
Barça drew 1-1 with Napoli in the first leg and the winners of this weekend's game will progress to the final eight in Portugal later this month.
On Dembele, Setien continued: "The kid's been working hard but he's still not completely confident (to return). It's normal. We could have given him some minutes but we felt it was better to wait and not force it. He will keep on training and we will see how he does."