On the back of four successive victories Saints had gone into derby day in good spirits. But when Leicester ran in four second-half tries the hosts were brought back down to earth with a massive bump.
"When you get well beaten at home it is pretty embarrassing," Mallinder told BBC Radio Northampton. "Of course we'll always look for positives and there were players who did well and put their bodies on the line. Tom Wood was outstanding, his energy for 80 minutes was high, and there were a number of players who showed good energy, but we didn't put it together as a team and against a good side like Leicester that's something you've got to do.
"At 12-3 at half-time we'd been put under pressure but we thought we were still in it. But fair play to Leicester, they got stuck into our set piece and played some good rugby.
"Defensively we were poor and conceded tries. It's hard when you are defending, but you've got to stick at it. Going down to 14 men didn't help, especially when we committed too many men to the breakdown.
"Once you go a couple of scores down you have to start forcing the play. You have to start playing a bit too much and that's when they scored their third and fourth tries."
Stephen Myler's second half try had threatened to pull Saints back into the contest
"You think you're going to get back into the game, and had the conversion gone over we'd have been seven points down," Mallinder added. "But it wasn't to be, and a lot of credit to Leicester.
"We created a lot of momentum over the last four games, but that's gone now, so we've got to regroup, get back and we know what we've got to do against London Welsh and Sale."