Just two weeks ago the Steelbacks were sitting top of the North Group table, having claimed 11 points out of a possible 12, and looking certainties to reach the knockout stages.
But they ended the campaign with four straight defeats, with the final nail in their qualification chances coming at Grace Road on Friday night when they were beaten by one run in a thriller.
A win would have seen the Steelbacks move back up to third with a real chance of progressing, but as it is they stay fifth and, having completed their North Group campaign, their Blast bid is over.
"We lost the game by one run, and it is disappointing, but the over-riding disappointment is that we were sat top of the league four games ago, and we haven't qualified," admitted Sadler, talking to BBC Radio Northampton.
"That is not acceptable, we should have qualified and we haven't, that's the bigger picture."
Sadler pointed to the first of those four defeats on the spin at Derbyshre on June 21 as the turning point for his team, feeling that was the night the team lost its momentum.
"It is a fickle game, so we try not to get too up when we are winning, and try not to get too low when we are losing," said Sadler.
"I think for me, in the Derby game we were poor in the field and we were poor with the ball that night.
"Looking back, that was probably a bit of a momentum shift, and momentum in this format is so important.
"When you have got it you do everything you can to keep hold of it, and if you lose it it can be sometimes tough to get back as we have seen in these past four games.
"The lads are hurting, we are all hurting and it is disappointing.
"We will reflect on it, and there are a couple of little bits we could have done differently, but over the course of the comp we have played some magnificent cricket."
And he added: "As a team we have done well in this competition, in the first half, the first two thirds, we were flying high.
"Everybody was performing, the batters were batting well, the bowlers were bowling well, we were fielding like absolute jets, but then something didn't quite click in the last four games.
"We will reflect on it when emotions have died down, and we will come back stronger."
The match at Grace Road on Friday was packed full of incident, and there were some unsavoury scenes when Jimmy Neesham was run out in the final over.
The Foxes players were unhappy with what they saw as gamesmanship when Neesham deliberately ran out Lewis McManus to keep the strike, and when he was run out himself a couple of players let their feelings be known.
Sadler felt the actions of the home players, and also some members of the crowd, weren't ideal.
"There was a bit going on out in the middle, and there was a bit going on behind us as well, so I don't know where the security guards were," said the Northants head coach.
"Look, we don't mind a bit of needle, and we don't mind a bit of passion when we play, and the emotion of it all, it is why we play and it's great.
"But it probably just went a bit over the top to be fair."
Northants don't have too much time to lick their Blast wounds, as they return to action on Sunday to play India in a T20 friendly at a sold-out County Ground (start 2.30pm).