It's a tough one, & as @Tomaszksays this spot is between Buendia/Coutinho/McGinn (none of them feel like natural wide players which is a whole different problem). I know you'll probably disagree, but I'd probably go with Buendia at the minute -- though I'd be rotating all 3 of them pretty consistently. But I think it's clear that this is the first position that needs to be upgraded in January.
Edit: I'd also be tempted to look into playing either Watkins or Bailey out here as well but then that requires additions to play in the front 2.