Redknapp said: "The defending for the goals on Sunday was dreadful. With 20 minutes to go I couldn't see us losing. It was even stevens. Then Tim Cahill came with a free header because we didn't track him.
"It's no surprise he does that. We worked on it before the game. So for Sulley Muntari to track him halfway and then switch off was frustrating. We had a dreadful start, got back into the game and did that.
"We have to work on it because if we think that was tough just wait until Saturday. That's going to be even tougher. Everton are a strong team but United are on another level."