I just don't think, in real time, Walker 'impeded' Sterling. He got an arm on him to put him off, but it's a contact sport. It's easily could have been given but this whole idea that any contact equals a foul just isn't correct to me. If Walker physically grabs his arm and pulls him back, foul. If Walker is trying to catch up to him, almost gets there and tries to put him off, not necessarily. Sterling's shot was made more difficult for sure but that's the whole point. Players get their bodies in the way all the time.
There's no attempt from Walker to play the ball, but it's not a tackle that's gone wrong. It's physical contact. Defenders aren't just going to let players have easy shots (unless they for Emirates Marketing Project!)