United finished their SPL season in sixth place, but they had no answer to the champions for whom Georgios Samaras scored twice, Kris Commons and Anthony Stokes adding the others.
McNamara said: "It was disappointing. I thought we lost cheap goals.
"Celtic's biggest threat is set-pieces and we gave ourselves too much to do by giving away silly free-kicks.
"Today doesn't change anything, although you want to finish on a high for the fans. I know what we have to improve on to become a better squad next year."
United skipper Daly is leaving the club to join Rangers, subject to passing a medical - and he was given plenty of stick by Celtic's fans all afternoon.
McNamara, who has signed Raith Rovers striker Brian Graham on a two-year deal, praised his outgoing captain.
He said: "Jon has been a fantastic signing for Dundee United. I think he will go down as one of the greats at this club."
Daly added: "I have done a medical so I am just waiting to hear back on that, and if that is all okay then it should be done and dusted.
"I think they were quite pleased with the shape of me. They want to scan my right knee because of the problems I had with that so I don't think it is going to get read on Monday.
"So we will wait and see how that comes back."
Daly admits his last game at Tannadice was emotional. He said: "Of course it was, I've had six and a half years at a fantastic football club.
"I am grateful to Craig Levein and Peter Houston for taking a chance with me by bringing me up from Hartlepool. It's always sad to move on but it is the right time."