Either don't train your loaned players so hard, or loan out less players.
The use of rests for loaned players is to benefit the manager using that player, NOT the manager who loaned out that player. Loaning out 8 players is a totally fine strategy, but if you're loaning them all out with low stamina, and then you get them back all refreshed, you're getting an unintended benefit that was never supposed to be the purpose of the system.
Personally, I think they could easily have gone less than 5 (perhaps 3), so they are being very generous with allowing 5. I suggest developing a strategy to better fit the new rules, rather than try to get back the old exploit.