Manager Jackie McNamara admitted Dundee United's defending was "embarrassing" in the 6-1 defeat by Celtic.
The Tangerines were three down by half-time and lost two more before John Rankin pulled one back.
Celtic responded with Jo Inge Berget netting his second to add to goals from Jason Denayer, Kris Commons, Stefan Johansen and Anthony Stokes.
"It just shows you if you don't defend properly at set pieces, Celtic will punish you," said McNamara.
"They've scored four from set pieces, they've not had to work hard.
"The third goal especially was self-inflicted with a straight pass. They intercept it and then he [Johansen] slots it home.
"We've not made them work for it, which is disappointing for me.
"It's not been like us so far [this season], we've looked solid enough. It's something we'll need to look at on Monday."
United won their first two Scottish Premiership matches of the season and travelled to Celtic Park as league leaders, with Ronny Deila's side having played a game less.
"I was confident before the game," explained McNamara.
"We had a good support down today and I was genuinely optimistic about the match.
"It doesn't matter who you're up against. If you don't defend set pieces properly and do your jobs, you're going to be punished.
"It becomes a lot harder when you're playing against the quality of Celtic because everybody wants the ball, the confidence is up and they have the players to hurt you."