The Leeds boss has been discussing the 21-year-old forward.
Indeed, plenty of excitement met the young forward's arrival at Elland Road earlier this summer but he is yet to feature for Rosler's men in the Championship.
The German is impressed with the 21-year-old, however, but is simply giving him time to adjust so that he can forego playing the odd match when not yet in peak condition or form.
Rosler is quoted as telling reporters: "It’s often the case that when you come from a different league you come from a different level. Lee surprised us because he took to this level relatively quick in the beginning.
"As a young player sometimes you have to be sure you’re patient and it’s important to throw him in for his first game when we know he can really do himself justice and when he can build his confidence. That’s what it’s all about.
"He accepted his role and he’s been on the bench for every game. But I definitely noticed that performance on Monday. I’m driving all the time on competition. Lee put pressure on his strike partners."
If Erwin can make an impact on the first team in the coming weeks, the timing will be relatively handy. Mirco Antenucci and Chris Wood are forming a good partnership but, if one or the other tires or gets injured, Leeds are light in terms of in-form alternatives.
Even if it simply comes down to tactical options, meanwhile, bringing Erwin on will be a big plus for Rosler - especially with United still looking for their first win.
Naturally, signing a new forward would help matters significantly. But that's far from guaranteed this late in the transfer window.