Tuchel told reporters: “I think it gets more and more difficult in today's game, Cristiano took a step into a league which is not as competitive as the Premier League or Spain - where he played before - as part of keeping his performance up. I think he is always the exception to the rule, we cannot go now from Cristiano Ronaldo and think everyone can play until 39 and 40 in today’s game.
“It will be very demanding, professionalism is a given in today’s game, otherwise you have no chance and Harry has this natural ability to score goals.
“He scored his 450th goal in the Andorra match, it was a very decisive goal, a very important one and there are at the moment no signs he cannot do it at a high level. How long he can do it, I have not a crystal ball, let’s see. He is in a club where they play dominant football, I think that can suit him.
“Just the outlook until the World Cup, he knows that it is a crucial year for him, also in terms of fitness and being ready to compete in the World Cup in the heat, in the humidity.”