Arsenal manager Mikel Arteta says Kai Havertz has undergone a "transformation in his body" while being injured and claimed he "destroyed all the metrics we had in the gym for the last 10 years."
Havertz has been sidelined since February after tearing his hamstring during the Gunners' warm-weather training break in Dubai but could return in Sunday's Premier League home game against Newcastle United.
It was initially unclear whether Havertz would feature again this season but he has impressed club staff with the pace of his rehabilitation, while some fans suggested he looked more muscular during an opening training session held at Emirates Stadium in midweek.
"There's been a transformation in his body," said Arteta. "The last three years in his career have been so condensed that he hasn't had time to do that and this has been a window and he really wanted to use it.
"To be fair, the staff and how they have explained the opportunity that is within with his physique to go to a different level and the things that he can improve and it can even help him mentally as well as being more effective as a player.
"He got on board with that, he worked hard, got on board with that, you can see his presence and I think he destroyed all the metrics that we had in the gym for the last 10 years.
"Power, acceleration, muscle mass, the way that he has increased certain capacities in his body, his agility, he's been incredible. I was talking to the medical staff the other day and they were talking about that.
"It's been a phenomenal effort from all of them under the circumstances. Last home game is always a special occasion and it is our chance to wrap up Champions League qualification.
"For someone that is not in the team to have that mindset, the only thing he has done is raise the standards in the building because the other ones in the building who are injured were looking at him, 'I'm going to look bad if I don't do the same' so that's part of the culture.
"And very grateful because when the players that are not playing are doing that, it's just raising the bar for the next one to come. If he is not at that level he is going to get exposed so that will make us better."
Arsenal want to bring in a striker this summer with sources telling ESPN that RB Leipzig's Benjamin Sesko, Sporting Lisbon's Viktor Gyökeres and Newcastle's Alexander Isak are under consideration -- placing Havertz's role in the team in doubt.
Havertz is still Arsenal's top goalscorer in the league with nine to his name so far and Arteta said: "One of the biggest qualities with Kai is his versatility and his goal threat whenever he plays and he's so intelligent. He will play as a No. 9, he will play as an attacking midfielder and he will be super important for us as a team."