The Leeds boss has been discussing the 21-year-old summer signing.
Indeed, the Leeds boss says Alex Mowatt's injury was the reason he didn't include Erwin on the bench against Brentford.
Asked if the 21-year-old is far off a first-team breakthrough, the German told BBC Radio Leeds: "No, no, he's pushing. For me, I had to make a decision about putting another striker on the bench or taking another midfielder. Because of the problem with Alex Mowatt, I took two midfielders."
Erwin's low-key start to his Leeds career can no doubt be considered a disappointment. At 21, the youngster clearly has plenty of time to make an impression.
But, considering the lack of options up front at Elland Road so far this term, as well as a relative lack of goals from Rosler's men, Erwin would surely have hoped for an opportunity by now.
If it's down to his fitness, that's one thing. If the youngster hasn't been picked yet due to his ability and form, though, all concerned may be feeling a bit frustrated.
With midweek action coming up against Ipswich Town, now may be the perfect time to at least give Erwin a chance off the bench. If he makes an impact from there, he could soon became a very useful tool indeed for United.