This might be weird coming from a Hannover-fan myself, but in case DDG is leaving why shouldnt United go for Zieler?
He is a United youth product after all, counts as home-grown, is still realtively young for a gk with just 25 and is the third german nationalkeeper as of yet (which is quite a thing) . Furthermore he wont cost a lot, like 8-10m and is already comfortable with United and knows the older teammates.
I count Leno as better than him, but Zieler comes with tons of advantages and has an error-count that low that it could make many famous goalkeepers jealous. He has played almost 200 matches in the league straight, never suspended, never injured, never dropped, unconstested league record. He doesnt make the same spectacular saves that DDG or his german gk-collegues do, but unlike them he also never lets one in that should have been saveable and is a solid reliability.
This way United can fill a spot in the squad with a homegrown player that is not just a squadfiller to match the rule, throw one of the unused homegrown players out and can get another foreign non-homegrown fieldplayer in addition to their squad without endangering the homegrown-rule. Furthermore it probably pleases many United fans to see a youth player returning to their squad who made it realtively far, as the last generations of their youth werent exactly spectacular. Kinda adds a little identity back.
If I was LVG I would defo keep this idea in the back of my head.
As for ourself, loosing him would be ok, because good gks kinda grow on trees in germany.