Of course he should leave Ings, he isn't as much an imminent threat as the whole person about to make an uncontested run to our goal. There were 2 or 3 players who could have caught up to Ings, there was no one who could have held up Watkins. He's meant to stay with Watkins, it's not even on the same plane of danger. The latter is an uncontested one on one situation. You know how cracked his decision making must be not to even realise this? "I let the striker run because I was worried about the player 10 yards away", the answer to this is in what followed his decision, he got caught in between both players in no man's land but not before he had so kindly played Watkins onside and rolled him a red carpet to our goal.
Edit Oh Lord, I even just saw the goal again and it's worse than I remember. The third man wasn't even part of the play, he was coming from an offside position. Maguire really sold out on good defensive positioning because an offside player cracked his focus. There's that brilliant tactical acumen that allows him to go round tackling teammates.