Saints have suffered some big defeats in Europe during recent years, but none as bad as this.
Leinster, Ulster, Clermont and Racing 92 have all thrashed Jim Mallinder’s men in times gone by, but this defeat to Castres was the worst of the lot.
Well, for a start, the 34-point defeat was the biggest Saints have endured since Mallinder took charge of the club in 2007.
And at least in the other eviscerations in recent years, Saints could point to an opposition outfit who were laced with star names.
Against Leinster, at least they were facing a team in full flow, packed with Ireland internationals, who ripped them apart in a 40-7 demolition.
And Ulster also had big names of their own when they came to Franklin’s Gardens and dished out a 25-6 defeat.
Saints were never expected to get anything from Clermont in the 2014/15 quarter-final, and they showed why, suffering a 37-5 defeat to a superb side.
And Racing were another cash-rich club, with the likes of Dan Carter and Mike Phillips pulling the strings in a 33-3 win in Paris last season.
But this Castres team is far from vintage.
Saints may have made them look like the All Blacks, but they are struggling in the Top 14, in which they sit 11th, and were easily beaten by Leinster in Dublin last weekend.
This time though, they sensed weakness, and they pounced.
A period during the first half aside, when Saints spurned a plethora of chances after going for the corner, Mallinder’s men were never really in this.
And they even lost a Rory Kockott sin bin period 7-0 to add to the agony.
The second half was then a painful procession, with Castres hungrily seeking the bonus point as Saints lay wounded on the Stade Pierre Antoine turf.
And, predictably, the home side, who were claiming just their fourth win in 10 matches this season, not only got the fourth try they wanted, but a fifth also followed.
It was so tough to see for the Saints supporters who had travelled to see their beloved side.
Again the typically boisterous French chants and ear-pounding drum beat echoed around the ground.
And again, Saints could do nothing to stop it, suffering yet another away defeat to Castres.
And the atmosphere on the fans’ coaches on the way back to Carcassonne told the tale.
For the players and management, there will be a similar sobering feeling.
But, unlike the fans, they can do something about it, and they will have to ensure that this humbling defeat is not repeated in the four Champions Cup pool games to come.
If they can look back on this day and say it made their campaign, then it will be worth it.
But for now, it will hurt. Badly.
It will be especially galling for the forwards, who fronted up to monstrous Montpellier last week only to get crushed by Castres a week later.
Saints rotated in a bid to freshen things up, but this looked like a jaded performance, mentally and physically.
They still haven’t won back-to-back games this season, prevailing in just three of their eight matches in all competitions.
And for a club with such high standards, that is nowhere near good enough.
The constant cycle of bouncing back from a defeat only to suffer another one straight after has become vicious.
Saints have to get some momentum like they have in years gone by.
They now have a run of games which should help them achieve their aim.
But the days of looking at the calendar and expecting victories have gone - for now.
Now Saints have to take every team on their merits and combat them accordingly.
Friday’s Aviva Premiership game against Gloucester has only got bigger because of what happened in the south of France on Saturday.
And if Saints don’t win against the Cherry and Whites, there will be more dark clouds gathering over the Gardens.
The positive is that is only early in the season.
And Saints can cling to the fact they they have summoned big reactions in the past.
But they will need to produce big displays week in, week out if they are to turn their fortunes around after a tricky formative stage of the season.
A siege mentality will probably be instilled after this defeat.
The players and coaches will listen to the criticism and use it to add fuel to the fire once again.
But at the moment, there is only the odd flicker of light from a tough start to the campaign.
And Saints desperately need a spark in the coming weeks.
If they don’t get it, the season could peter out at a very early stage.
How they rated...
One of the few Saints starters to emerge with some credit as he kept battling until the end, making some big tackles to stop more Castres tries being scored... 6
Scored Saints’ late consolation, but didn’t have too many more chances to attack as Saints struggled to get on the front foot... 5
A big ask for the youngster to come in and start his first European game in France, but he wasn’t overawed... 5
His big boot was used to gain territory from penalties, but he wasn’t able to influence the game in an attacking sense and had a tough task to stop Castres... 5
Came off with a hand injury during the second half after making a good turnover and a vital tackle in the first period, but wasn’t able to influence the game... 5
The Saints half-backs couldn’t get any control and this was not a performance they will have enjoyed... 4
Tried to spark things early on, but his passing let him down at times and this was a tough day... 4
The Saints forwards lost the battle to their imposing hosts and this was a difficult day for the front row... 4
Made his first appearance since the win at Bristol on September 11, but coming back in against a fired-up French team was never going to be easy, and so it proved... 4
Can never be accused of not putting in the effort, but Castres had Saints’ number up front and the prop’s day was a tough one... 4
Like Hartley, this man also made his return from injury, but he was unable to assert any of his usual dominance... 4
Came off early in the second half after trying to continue his strong start to the season but struggling to contain the French physicality... 4
Made more tackles than any other player on the pitch as he desperately tried to help Saints salvage some pride from proceedings... 5
Caught the eye so often last season, but hasn’t had the run in the team he would have liked this season and is yet to get back to his best... 4
Never shirks a challenge and gave everything he had to get Saints moving, but it wasn’t enough on this occasion... 6
Replacements (who played more than 20 minutes)
MIKE HAYWOOD (for Hartley 45)
Showed some fight when he came on, making one break and frantically trying to help Saints get a foothold in the game... 6
API RATUNIYARAWA (for Paterson 45)
Not much the big man could so this week as he ran into plenty of brick walls, with Castres looking strong... 5
LEE DICKSON (for Groom 45)
Had little ball to work with as Castres pushed on in the second period and this was not a cameo he will have enjoyed... 5
ETHAN WALLER (for A Waller 53)
A tough game to come into as Castres had their tails up and the prop was fighting against the tide... 5
PAUL HILL (for Brookes 53)
Another man asked to come on to try to stem the flow of points in a team effort that was sadly lacking... 5
AHSEE TUALA (for North 53)
Unable to have any influence as Saints failed to get any go-forward and gave their backs little to work with... 5
JAMIE GIBSON (for Picamoles 54)
Always a steady enough performer, but the game had gone a long time before he came on... 5