Though his dribbling and close control are certainly wonderful, he's been so effective at changing pace to beat defenders this season. He waits for defenders to approach him or commit to a challenge and then with either a body feint or burst of pace he's off in a different direction, often using their inertia against them. It seems to be a simple skill on the surface but watching a player use it almost to perfection is such a joy to watch.
He's been playing well and I'm loath to criticize him, but in games where he's playing 60+ minutes his movement and work rate drop off at times, and for that reason I understand why Mourinho might opt for someone else in a tough contest against a team that will likely have a fair share of the possession.