Arsene Wenger admitted that he is at the mercy of Alexis Sanchez’s honesty as he tries to judge whether the Chilean is ready to play against Manchester United tomorrow afternoon.

Sanchez did not return to London until Thursday afternoon, and will not be assessed by Wenger until Friday morning. That means that Wenger will have to take on trust how ready Sanchez is, after his star striker played through a hamstring scare over the international break.

The problem for Wenger is that Sanchez is so enthusiastic that he will say he is fit even if he is not. Wenger admitted at his press conference yesterday morning that Sanchez would be desperate to play, regardless of the actual state of his hamstring. “He is always okay,” Wenger joked. “It is a very difficult one, because I am sure he will come back saying he is all right.”

“What I will consider is the risk of injury because he played while recovering from a hamstring injury,” Wenger said. “I’ll consider the way he feels as well. On that front you depend on the honesty of the player, how they feel and how they recover.” Sanchez let Arsenal know by text message after the game that he was feeling “in good shape” for his return to his club.

The fact that Sanchez played 84 minutes for Chile on Tuesday night, and scored two goals against Uruguay, would suggest that he is fit. Wenger slept through the match itself, but having seen Sanchez’s highlights back, he was confident that his best player would be able to play some part on Saturday.

“I fell asleep,” Wenger said. “I have watched the goals and he looked very sharp. I will watch the game later. I know him so well, like all my players that I observe every day, so I will be able to see if he was playing with a restriction or not. But the first goal he scored he ran from the halfway line. If you are really injured with a hamstring you cannot do that.”

Wenger certainly had real reservations about Sanchez playing on Tuesday, after he had limped out of training the previous week. But it was an important game in Chile’s bid to qualify for the 2018 World Cup in Russia, and Wenger was powerless to stop him.

“You have to trust the medical people from Chile,” Wenger said. “I completely understand that Alexis wanted to play in qualifiers - they’re not friendlies. Chile are not in a fantastic position to qualify for the World Cup and I understand that Alexis is keen to play for his country in such an important game.”

Given that Sanchez will barely be able to train with Arsenal before the squad travels to Manchester on Friday, the likelihood is that he will be on the bench for the game, with Olivier Giroud starting up front.