The Uruguay striker, who picked up a thigh injury last month and missed his club's 2-0 win in the first leg at Old Trafford, has taken part in the last two training sessions.
"He took part in the last two training sessions in full, but we are going to wait until tomorrow," Tuchel told a news conference on Tuesday.
"We have decided to wait and make a decision on his participation in the game tomorrow."
Tuchel's counterpart Ole Gunnar Solskjaer said scoring the first goal would be crucial for United to overturn the deficit -- a scenario that Tuchel did not rule out.
"Our goal will also be to score the first goal," he said with a smile.
"But we have to play with calm and confidence, be aggressive and compact. We also have to be careful but not scared.
"They might score first, but we have to be ready to react if that happens."
(Reporting by Julien Pretot; Editing by Hugh Lawson)
