As a club, we should be doing all we can to keep him.
But we need to think of the player too, and as a player he'd probably want to move:
- chance of replacing Lammens is incredibly slim. He'd have limited chance to impress, and as a 2nd keeper any blunders will be so much worse.
- he's still young, and playing every week will definitely develop him more no matter how many cup games he plays. If he's outright 1st keeper at a smaller club, he'd be able to develop with less pressure and more forgiving circumstances. If he does well for a few years, he's still young enough to earn a move to a huge club anyway.
- as for a loan, why would he prefer that to a more stable and secure move to be a guaranteed no1 at a relatively good club (he's at least moving to a championship/lower prem level club)
Frankly, in his shoes, would anyone choose to stay? It's highly likely staying as a 2nd will only hinder his career, since if he plays well enough for a smaller club, he'd still get his big move and play at a top club for 10+ years. If it doesn't work out, he's still getting valuable playing time to maximise his potential.
In this case, it's better to just part ways amicably, try to get as much cash as we can, and wish him the best. (buy-back clause if possible)