The Recreation Ground was sold out on Saturday as Bath went in search of a win that might salvage Heineken Cup qualification from a decidedly mixed season.
Instead, it was Northampton who bounced back from a difficult week in which skipper Dylan Hartley and flanker Calum Clark were handed bans of eight and 32 weeks respectively, claiming a comfortable win with a penalty try and another by former Bath hooker Andy Long.
Ryan Lamb kicked 11 points and Stephen Myler a conversion while Bath's points came from two Stephen Donald penalties.
Forwards coach Haag said: "We didn't perform as a team anywhere near where we needed to. We were way off the pace, looked very disjointed, lost a few set-pieces early doors and didn't get into any rhythm.
"At the start of the second half we actually played some decent rugby but we made a couple of errors close to their line and the pressure was off them. For the last 20 minutes they dominated and we were playing catch-up.
"From our point of view it was totally unacceptable, a very disappointing performance, especially in front of a very big crowd.
"We expect more as a team, and I include the coaches and the playing staff, everybody. Playing at home you expect more than we gave out there today so we owe everyone an apology."
Asked if speculation about a new coaching team to be installed at Bath during the summer had unsettled the squad, Haag said: "You could easily use that as an excuse, but no.
"We were pretty focused on the game today, but so were Northampton and they have had disruptions this week.
"We didn't play very well today, in fact we were awful. It's nothing to do with anyone else outside the changing room. We have to put it right, to be mentally tough and strong to come through this "
Northampton director of rugby Jim Mallinder paid tribute to his players' resilience and spirit after a difficult week.
He said: "In a Premiership campaign you get lots of ups and downs. What you have to do is keep focused on the job in hand.
"We've done that all week in terms of our training and the 23 who've been playing today and I think that showed today, looking at the spirit of the team, some of the accuracy.
"Our set-piece was outstanding in that it deprived Bath of any line-out ball in the first 20-25 minutes."
Speaking about the suspensions meted out to Hartley and Clark, Mallinder said: "It's disappointing that it happens, you don't want it to happen.
"You've just got to have confidence in your ability and we do. In terms of what has happened, and the outcome, you just pull yourselves together."
Mallinder added that he will consider on Monday whether to appeal against the length of the ban imposed on Clark, who was found guilty of causing a serious arm injury to Leicester hooker Rob Hawkins.
"We'll support him as we will Dylan," Mallinder added. "What you can't do is take away that competitive edge. Sometimes, whether it's through frustration, players can go over the top."
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