We have Chilwell and Maatsen for LB/LWB so getting rid of Cucurella who's on high wages makes a lot of sense, even if it's only a temporary move.
Reportedly we're after a £7M loan fee and United have offered £2M. Let's say the difference is split in the middle and the final loan fee was around £5M. That fee as well as having United paying his salary for the season would be a wonderful deal for us. Basically he'd only cost us £4M for the season as compared to the £18M he currently does (amortisation + salary).
Next year his book value would be lower and chances are he'd have redeemed himself a little bit playing for you lot so it could be much easier to get rid of him on a permanent move then, even if you didn't buy him.