The tough talking German has been keen to instil a strong work ethic throughout his squad and it was clear against Bolton on Sunday that some of the players were struggling to keep up.
Rosler's tactics demand the highest level of fitness from the players and the new manager even went so far as to leave Grant Holt at home for last Thursday's European game to work on his fitness.
“One thing that is showing itself at the moment is that our boys are not 100 per cent fit,” Whelan told the Wigan Evening Post. “I had suspected that when I’d been watching them train under Owen, and what he was doing with them.
“It’s going to take the new manager at least a couple of weeks to get the lads where he wants them. And you could see from Sunday’s game that one or two were dropping dead before they came off the field.
“Uwe has got a lot of work in front of him, but I like what I see so far.”
The Latics presently sit eight points away from the play offs but a victory against managerless Sheffield Wednesday tomorrow would see that cut to just five points.