He didn't fail to track any runner from midfield. When Liverpool won the ball, Blackett was out of position (through little fault of his own, it was a shocking pass from Marnick) and Thorpe was left trying to deal with Sinclair and the runner from deep. Had he left his area to cover Sinclair, Sinclair could easily have played midfield-guy in one-on-one with Valdes. He kept his position as Blackett was on his way over to Sinclair as soon as Liverpool won the ball, and Blackett might have gotten there, too, had he been proactive and sprinted as soon as Liverpool gained possession, rather than reactive by jogging until the pass was played.