Lyon boss Pierre Mignoni admits the opening 20 minutes cost his side the match as they were beaten 25-14 at Franklin's Gardens on Sunday.
And Mignoni praised Saints fly-half Dan Biggar for his 'exceptional' display.
Bigger kicked 20 points in a flawless showing as he steered the black, green and gold to a crucial Champions Cup win on home soil.
Rory Hutchinson registered Saints' only try of the game on 19 minutes.
And Mignoni, who had seen his Lyon side win eight of their nine league games going into the European encounter, said: "The first 20 minutes are the opposite of what we wanted to do.
"Then we put things in order, we won the second period, but that's not enough.
"There were positive things, like our scrum, which was strong.
"The second half reassured us a bit because we were really frustrated at the break.
"Northampton made the difference in the first 20 minutes.
"Strategically, they do the right things.
"Biggar had an exceptional match and they keep you in your own half until you make a mistake.
"We lost lineouts and it's too much.
"You need to start matches quickly and now we must switch to the one next weekend when we play at home against Leinster.
"It's going to be a big challenge."