Early goals from the recalled Billy Sharp and Rudy Austin, who scored with a goal of the season contender, put United firmly in the box seat at 2-0 ahead after 19 minutes.
But Troy Deeney pulled one back for the visitors six minutes before the break, continuing his fine goalscoring record after Leeds and his strike partner Matej Vydra firing a second-half double to enable the Hornets to take the points.
The Czech Republic born striker netted the winner nine minutes from time for the Hertfordshire outfit, who have now lost just once in nine outings to Leeds.
Redfearn, who said Austin (thigh) is an early doubt for Wednesday’s home encounter with Ipswich Town, said: “It was disappointing after being two-nil up. I thought we played really well in the first half, I have to say.
“We created chances and took our chances well. But when you look at the first goal for them, it was poor defending.
“Then with the goals after, the defending was poor. You can’t defend like that and you have got to defend better than that.
“We were always in the game because we were playing well enough, but the game got too open and we needed to be more thorough in midfield and it was disappointing.
“Maybe the game-changer was Rudy’s chance just after half-time. If that had gone in and made it 3-1, then obviously it is a mountain to climb for them. But at 2-1, they were still in with a chance.
“I just felt we were really sloppy at the back, which is unusual and unlike us.
“We just have to defend better and be more thorough when we are attacking and going up the other end. I thought it was too easy to cut through us when the ball changed hands.
“You are looking for your experienced men at the back. You have your skipper there and an experienced guy at centre-half in Sol (Bamba) to go and organise. We just had an off-day.
“It looked too easy. I said before the game that with the nature of the side they had, we were going to find ourselves defending one v one because they break quickly.
“It was up to us to make good decisions and stand up and hold players up and allow the cavalry to get back and recover.
“But I thought the decision-making was poor. It is a shame really as it put a downer on a half-decent performance. We played quite well at times and created enough chances and could have had three or four goals and it could have ended up five apiece.”