Tuchel admits the winger must "fight" if he wants find a way into his squad.
The German said, "In general nobody is for sale, and everybody is for sale.
"It depends in the end on what the player wants, how bad the player wants it, what we think, and who is ready to pay a certain amount of money, and then maybe everybody is for sale.
"Callum competes, he had a long-term injury and long-term problems to overcome so that's from where he suffered. I think he has still a long way to go to live up to his potential because the potential is huge.
"Having Raheem [Sterling] in can maybe give him the extra boost to step and see what is needed on these kind of positions to reach the level that he wants to reach and that he can reach. At the moment he's our player and he needs to fight and fight."