Amass took part in training before their trip to Istanbul as he looks to make his first appearance for the club this season after enjoying a breakthrough pre-season tour.
The 17-year-old is reminiscent of Luke Shaw in the way he patrols the whole left flank and has a great delivery of the ball into vital areas as well.
Fellow left-back Tyrell Malacia was also part of the training group and along with Luke Shaw took part in an individual session as the pair look to return to full fitness after long term injuries.
Manager Erik ten Hag has admitted he is excited to face Mourinho on Thursday night.
"It's a big game for both of us," he said.
"I really enjoy facing him and playing against him. He has always good teams, he is a winner - he (has) won so many trophies - I think he is an example for many, many managers."
UNITED'S TRAVELLING SQUAD:
Goalkeepers: Altay Bayindir, Tom Heaton, Andre Onana.
Defenders: Harry Amass, Diogo Dalot, Matthijs de Ligt, Victor Lindelof, Lisandro Martinez, Noussair Mazraoui, Habeeb Ogunneye.
Midfielders: Casemiro, Christian Eriksen, Manuel Ugarte.
Forwards: Amad, Antony, Alejandro Garnacho, Rasmus Hojlund, Marcus Rashford, Ethan Wheatley, Joshua Zirkzee.