The head coach explained that the striker was finding it a struggle being at the club and not being involved.
Erwin was sent on loan to Bury on a short term deal, and made his debut last weekend against Wigan Athletic.
The German has not yet judged Erwin to be ready to feature for Leeds, and hopes he will be better off for first team football.
He said: "I thought Lee needed a different playing level than under-21 football.
"He played already in the SPL, and came to us with high hopes, he was very good in pre-season, because he had a short holiday, but others caught up.
"It's been very hard for him not to be involved on matchday. We need to keep his morale high, to give him men's football on a Saturday."
Erwin's loan to League One side Bury is just for an initial 28 days, but Leeds could depend to extend it to give the player more playing time, if they do not have use for him right away.
Fans will have their views on the quality of the Scottish Premier League, where he played last season for Motherwell, but it is more competitive than under-21 level, and there must be little more depressing for a footballer than sitting around on Saturday with nothing to do, while players he has trained with all week head on to represent the club.
It was a scenario which Nicky Ajose found tough last season after failing to make the Leeds team. He told Crewe's website after signing on loan that he had to spend Saturday afternoon's training rather than playing, describing it as 'difficult.