Kristian Woolf admitted his side were well below their usual standard as the Betfred Super League leaders suffered a first defeat of the season at Catalans Dragons.
Out-of-sorts Saints looked jeg-lagged in the face of a powerful Catalans performance led by man-of-the-match James Maloney and were beaten 20-16.
A late revival by the visitors led to a frantic finish in Perpignan, but Saints could not quite finish the job and the back-to-back champions crumbled to a loss at Stade Gilbert Brutus.
"We're all disappointed," St Helens head coach Woolf said. "That performance wasn't anywhere near our standards and we'll have to get things sorted before our next game.
"We're very frustrated. I liked our fight at the end and we had patches of that in the first half, but our 20 minutes after half-time was really poor and we backed that up with some poor defence too by our normal standards.
"That's where the game is won and lost. We had to show the fight that we had to get into the game in that early period in the second half.
"We want to play a bit of footy and our halves are in control of when we play and when we don't. The rest of us have to make sure we're connected with that. It's about execution and concentration and that's something we didn't do too well tonight."
The Dragons were hit by the sudden loss of star full-back Sam Tomkins and centre Samisoni Langi who both pulled out with injuries just before kick-off but young French replacements Arthur Mourgue and Mathieu Laguerre filled the gaps with flair.
Australian stand-off Maloney put Catalans ahead with an early penalty and then weaved his way through the Saints defence for a try on 26 minutes.
Full-back Lachlan Coote got Saints on the scoreboard with a penalty just before half-time but the champions buckled early in the second half as Dean Whare crossed.
Saints hit back with a Mark Percival try but Coote couldn't convert then Ben Jullien crossed for Catalans, Maloney converting from the touchline to make it 20-6.
Percival bagged his second try on 71 minutes then Regan Grace crossed to make it a four-point ball-game with just three minutes left.
The Dragons managed to resist a late Saints onslaught and left the supporters singing for the first time this year.
"I'm very happy with the team, they worked very very hard against a very strong team," Dragons head coach Steve McNamara said:
"Losing Sam Tomkins and Samisoni Langi before kick-off made things very difficult for us. We'd had a short week, coming back from Hull on Monday, so for us to cope with all of that it was a great performance.
"Mathieu Laguerre trained with our reserves this morning and he was expecting to play at Lezignan tomorrow, but he had a fantastic game, alongside Arthur Mourgue.
"The best defensive teams give themselves a chance to win the competition and we haven't been that over the last couple of years and it is something we acknowledge.
"Some of the decisions we have made with our squad this season have been designed to enhance that side of our game.
"I thought we were very good defensively against Saints tonight. I don't think we were too great on attack, but our defence won the game."