You're right, but I still think he should play easy games, just to test him and see if he's got it in him. But Digne certainly can play there.
It's true that the only reserve player I'd consider buying would be a versatile one, like Keita back in the days, but we should generally stop spending money on reserve players and start promoting them from within (ones who have potential and are good enough). In a 23-man squad, we should have 2 to play in each position, and one "number 12" (a player capable of playing in many positions). We already have Roberto though.
Buying talented players wouldnt simply be as a reserve option though, but as a potential successor to our starters. And the more versatile they are, the more playing time they'll get and the more easy the transition will be. That's why I dont like signing players like Digne who was bound to get very few games and be rusty and unhappy with the situation. Its been a long time since we've had any successes with La Masia, so I guess we're due for a successful player there soon.