Jim Mallinder admitted Saints had a ‘terrible first 25 minutes’ at Welford Road, but felt a disallowed try ultimately cost them dear.
Mallinder’s men were 21-0 down after Leicester Tigers scored three tries before half an hour had elapsed in the east midlands derby.
But Saints battled back, with efforts from Teimana Harrison and George North pulling them back to 21-17 behind.
Paul Hill then had a try ruled out after referee Wayne Barnes deemed there to have been a high tackle from Tom Wood on Peter Betham in the build-up.
Leicester kicked on, and three Freddie Burns penalties - the final one of which came from the last kick of the game - secured the bragging rights.
And Mallinder said: “We came back into the game in the second half.
“We had a terrible first 25 minutes and you shouldn’t go 21-0 down at Welford Road.
“But we came back into it, we really got some good momentum in the second half, we put them under pressure, we were playing down the hill and the disallowed try was a massive moment. If that had been allowed, I think we’d have gone on and won the game.”
Saints had levelled at 27-27 after Jamie Gibson scored against his former club and Stephen Myler landed a penalty and a conversion.
But Barnes awarded a last-gasp penalty against Courtney Lawes, giving Burns the chance to win it - and he didn’t turn it down.
“My reaction is clearly one of disappointment,” Mallinder added.
“I spoke to Courtney afterwards and he said he tripped. He did put his hand on Ben Youngs, but Ben Youngs is a professional and he knew what was at stake there.”