If the COVID thing is true and his fitness has considerably declined, it sounds like this won't be an easy fix and will be an ongoing issue for him.
Tatum of Celtics (NBA) caught COVID in 2020 and looked off for the rest of season but went back to his best the following season. Same happened to Saint-Maximin I think. A young athlete should be able to fully recover but it might take a few months. It should not be something to affect his long term career.
Not really sure of where COVID rumors about Sancho come from though, was he even reported to have had it and it affecting him significantly? This could just be a case of a player not looking after themselves during offseason and turning up out of shape. This happened before to many players.