Ramos, who was doing individual ball work in training on Sunday, has been out for over a month with a calf problem.
He took part in full training on Monday and has travelled to London with Zidane's squad, which would be a huge boost for the Spanish side.
Tuchel admits he expects Ramos to start the second leg but demanded his side don't "lose our heads" over his inclusion.
He said: "The main lesson is we can trust ourselves and have the right to be in the semi-finals.
"We should be self-confident, have absolutely no fear to face this challenge because we deserve to be there and can perform at that level again. This is the main lesson.
"Does Ramos play or not? This is a tough question because i don't know if he's fit or not. We'll see after training and when they give the line-up.
"Does it change for Real Madrid? Yes it changes them a lot. He's the captain of the most successful team in Europe in recent years.
"It changes a lot but we cannot lose our heads about this decision. I think that he will start, we will prepare for that and we have to make sure he can't do it alone."