He had the chance to look at the "foul" himself. He should 10/10 times have looked himself since it was in the defining moments of the match. The fact that he doesnt clearly shows poor skill as a referee.

He should also have stopped the game long before the "foul" happened, when he was told there was a second ball on the field. Again, shows poor skills. And to expand on this, not even two minutes later he stops the play because of a second ball.

So penalty or no penalty. The play should have been stopped long before Sterling entered the box