I'd be horrified if we paid 15 million for him. I've seen him play 4-5 times, and I've never been impressed. My thoughts:
He's not a winger at all. He's a left-footed attacking mid, except a pretty average one. He's young, hard-working, and decent technically - I'll give you that. But he has very little going for him otherwise - he isn't particularly quick, a high centre of gravity, which means he's ill fitted to be a winger, even a converted one. Has very little explosiveness on the ball, and average-to-decent passing skills.
Poor as a winger, and almost underwhelming as a playmaker. He's neat and tidy with the ball, passes okay, and has a decent left peg. That plus his age and attitude means he'll probably make a decent-ish player down the road. He is however by absolutely no stretch of imagination a world class talent, and if we spend 15 million on him, I'll be fucking appalled.
Right about now, he's almost like a South American hybrid of Henderson and Downing, with a similar price tag. Stay fucking clear unless you're getting him for 6 million, and want to develop him as a good young contributor for the future.
Did I also say that I think he's not suited for England at all - nothing to do with his stature or with his pace - he's just not quick enough on the ball, or someone who's clever enough to play the game 2 steps ahead in his head.