Newcastle Falcons boss Dean Richards felt his side's fitness was an issue as they let a big lead slip against Saints at Franklin's Gardens on Saturday afternoon.
The Falcons were 21-0 and 28-7 up before eventually being beaten 59-33 in the Premiership Rugby Cup semi-final.
Saints, who will now host Saracens in the March 17 showpiece, ran riot during the second half, taking their tally to nine tries on the day.
And Richards admitted that Newcastle were too fatigued due to fielding some players who have not had much game time in recent weeks.
“The first half was great to watch if you were a Newcastle fan and the second half was if you’re a Saints fan, and it was a game which had a lot of ebbs and flows," Richards said. "Sadly we came out on the wrong side of it.
“We were 28-7 up at one stage and were on fire, but we’ve got too many boys who are coming back from injury and hadn’t played for a long time.
"In that squad there were probably seven or eight who haven’t played for a while, and it showed on the day.
“In the first half-hour we got things right, our accuracy was good, but as soon as we tired we began to drop off.
"Northampton have been playing together a lot, and it showed as the game went on.
“The positives are that the things we worked on in training, we put into practice. To get 28 points so early on was testament to that, but out fitness levels and our conditioning levels I don’t think are quite right.
“The boys just aren’t fit enough, some of them mainly because they’ve not played for a while, and we’ve got to look at it very closely.”