The Argentina winger has already had a first week of training at Chelsea after joining on Monday from United in a loan to buy arrangement.
At the end of his first training session, he admitted to a member of the club's media team according to The Mirror: "I can't move my legs anymore, man," before adding: "I sprinted on the last one."
A confession which illustrates his state of fitness, while he was not selected for Argentina for this international break.
In London, manager Enzo Maresca clarified what he expects from him: "When we look for a winger, we look for someone who is strong one-on-one, who knows how to attack and create things.
"Jamie Gittens, who arrived this summer from Dortmund, fits that profile, and we'll see with Garnacho. He's a good one-on-one winger and he can create. That's my vision."