Krul pulled off a string of remarkable saves - including one from a point-blank Nemanja Vidic header - to prevent United netting a winner and securing all three points.
"He has been in top form all season and for me he has been one of the better goalkeepers this year," Simpson told MUTV. "He made some great saves and you need that when you're in the back-four. Sometimes you can't stop certain things so when you have got him in goal making saves like that, it is nice.
"If you look at the chances on Match of the Day, people might ask how United didn't win the game. I can see why they do score late goals - they are so clever and have got players like Rooney. You don't know whether you're coming or going.
"The offside decision against Hernandez in injury time was a relief but we kept going and managed to deal with their threat."