Dan Ballard says Granit Xhaka's arrival raised the standards at Sunderland to help them compete in the top half of the Premier League.
Switzerland midfielder Xhaka was one of several summer signings following the club's promotion from the Championship through the play-offs.
The 33-year-old, who joined from Bayer Leverkusen, was installed as captain, and his leadership has had a clear impact on a side that has settled right back into life in the top flight.
Sunderland are currently eighth in the table - just two points off the top four - following their 1-0 derby win against Newcastle on Sunday.
Ballard, who was in Arsenal's academy when Xhaka was at the Gunners, told Football Focus: "He's an amazing guy.
"I was a bit surprised when we signed him. I wasn't sure what it'd be like. I didn't think one person could affect the team the way he's done.
"From minute one, I think the standards really rose because he does everything to 100%.
"It's kind of like, well if he's got that amazing career and he's still doing everything perfectly, why wouldn't we? And I think it just sort of lifted the standards."
Sunderland have won seven and lost just four of their 16 league games, and Ballard attributed Xhaka's influence to that winning mentality.
"I mean literally everything, from the work in the gym to the standards, he puts out on the training pitch," added the centre-back, 26.
"You know we've just been outside, it can be cold and wet and but he's 100% focused and giving everything in training.
"So when some of us boys may be not putting that work in and you see him doing it, it's unacceptable for us.
"If he's doing it, he's won titles and played hundreds of games in the Premier League. Then why shouldn't we? It's lifted everyone."