Northampton boss Jim Mallinder branded his side's display "unacceptable" after they were humbled 24-11 by Wasps at Franklin's Gardens.
Wasps scored three tries in the opening 30 minutes as they won in Northampton for the first time since November 2005, leapfrogging the Saints in the Aviva Premiership standings in the process.
The home side had gone into the game on the back of successive Champions Cup wins that earned them a place in the quarterfinals of the competition, but they were well beaten by a Wasps team shorn of a plethora of stars including Joe Simpson, Elliot Daly and Christian Wade.
Saints director of rugby Mallinder, who saw number eight Sam Dickinson score a late consolation for his side to go with two Stephen Myler penalties, said: "We were hoping we'd turned a bit of a corner in Europe. We'd put in two good performances, scored some good tries and we needed to back it up.
"But that first half was unacceptable, our defence was poor, we had no linespeed whatsoever, we couldn't get our set-piece functioning at all, so we gave our backs nothing to work from.
"Just generally, we looked flat as a team. We made a couple of changes, as did they. You come off a couple of games in Europe and you look to freshen things up, but clearly their changes worked better than ours.
"We've had losses before, as every team does, and you really have to look at yourselves."
Dan Robson, Rob Miller and Josh Bassett all dotted down inside a blistering opening 30-minute spell from Wasps, who had scored 51 points in a fine win against Leinster six days earlier.
That early charge, which also saw Jimmy Gopperth convert all three tries to go with his early penalty, saw the visitors build a 24-3 half-time lead which they never looked like relinquishing in the second half.
Delighted Wasps director of rugby Dai Young said: "I couldn't be happier.
"We spoke about backing it [the Leinster win] up and there were a lot of players out there who haven't had as much rugby as they would have liked, but you're only as good as your squad and it's important that when boys get a chance, they stand up and take it. They certainly did that tonight.
"I thought we were excellent and from the word go, we really came out of the blocks. We thoroughly deserved our win.
"It's the first time in more than 10 years [Wasps win at Northampton] and hopefully it's not going to be another 10 years before we do it again. Overall, I couldn't be happier with the performance and the result."