It is a good move for the lad, however he should be banging on our doors by now, not pondering another loan deal, seems to me the lad is struggling to reach the level required to play alongside Virg et al, but some flowers take longer to bloom than others, so if Jürgen feels he can improve further who am I to argue!!
He was unlucky last season, Billy. They loaned him to Schalke and the lad quickly became a starter and their fans were praising him (yeah I did the thing where you go to Schalke fan forums and google translate his player thread
), but he tore a ligament in his ankle and spent pretty much the rest of the season recovering. He only came back for Schalke's last 5 games of the season, playing the full 90 minutes in 4 of those games (the first game back from injury he was subbed on for a 15 minutes spell or so).
I'm pretty sure that had he not got injured last year he'd be competing for a spot in the team this summer because I watched 2 of those first 4 games he played early on for Schalke last season (it was games against Dortmund and Bochum) and he looked pretty good. He was partnered with Maya Yoshida (Japanese CB that used to play for Southampton for quite a few years) in a back 4. I'd argue he was Schalke's best defender in both games. Quick, very calm, when the ball is at his feet he's very composed and has a nice range of passes and overall I feel like he reads the game well.
Another loan to the Bundesliga makes sense in my opinion. We loaned him to Preston and that went well for him even if he was playing as a wing-back. The next logical step is to loan him to a league where the level of football is up a notch or two from the Championship. Of course all of this should've happened last year, but its the cruel nature of football sometimes, completely wasted season for him last year. I'll be eager to see if he put on some muscle in the off season (I think it's something he needs) and how he looks overall on the pitch with Mainz.