I have mixed feelings on this move.
It is clear that Isla needs a run of games if he is ever going to get back to his Udinese form, and he won't/shouldn't get that at Juventus. If we sell him now we are likely to make a huge loss given his form for the past couple of seasons.
So maybe a loan to try to get his form back and potentially bring him back/sell at a higher price is a good idea.
I cannot work out if English football (brainless running, speed and tough challenging) is a good match for him. Probably a move to Spain would suit him best where he would have more freedom.
I see this as a bit of a gamble by Marotta, if he has a bad season we could really struggle to offload him next year. But it certainly puts him in the shop window if he does well.