Munster got the job done, just about.
It wasn't pretty and it never flowed but given their travails over the last month, boss Johann van Graan will be glad to finish this Heineken Champions Cup block with two wins from two.
They came up against an opponent here whose heart was never really in it but, who in the end were close enough to worry the Munster men, even if just momentarily.
Ben Healy, in for the injured Joey Carbery, kicked 14 points, while Jack O’Donoghue got their only try and that was enough on the day.
Castres president Pierre-Yves Revol called the situation that saw his team having to fulfil this fixture "absurd" after seven European ties involving Top14 sides were postponed by tournament organisers on Friday.
It is hoped that the games will be refixed, while Leinster's game at Montpellier was cancelled, however, the Irish side are expected to appeal this decision, not least after they were handed a 28-0 loss.
But Thomond, still reeling from yesterday's news that the sold-out derby against Leinster on St Stephen's Day would now have a 5,000 capacity, was hungry for European rugby and to hail their players, who have undergone a chaotic month after getting caught in South Africa as the Covid-19 crisis escalated.
A squad that featured 12 debutants went to Wasps and conquered the English side, returning home triumphant with a bonus point to boot.
Back in charge and overseeing his first game since he announced he would be leaving the club at the end of the season to join Bath, Van Graan brought in five new faces with Ben Healy starting at out-half with Niall Scannell, John Ryan, Jean Kleyn and Jack O'Donoghue packing down.
Over-excitable in midfield, sloppy at the lineout and ill-disciplined at the breakdown, Munster looked flat in the first half. After all the hassle and hype of the last four weeks, it was perhaps understandable.
They did manage to carve out three penalties over the course of the first period with Healy landing kicks in the eighth, 29th and 35th minutes.
The last of those came after referee Matthew Carley took an age to decide that man of the match Damian De Allende had lost control of the ball as he dived over the line.
It was a harsh call, with every replay seeming to confirm a good try.
The South African had been one of Munster’s better performers, enjoying space in midfield to stretch his legs but the hosts, who had won all of their previous seven home games against Castres by an average margin of 22 points, couldn’t make the breakthrough.
It was a slightly tetchy half, too, with Pierre-Henry Broncan’s side offering little in terms of attacking play and they would have been content to get in just six points behind at the break when a Munster tackler didn’t release.
Veteran out-half Benjamin Urdapilleta tapped over from the 22 and that was the last score of a strange half.
Sitting sixth in the French league, Castres were guilty of their own sloppiness at set-piece and were twice walked back 10 metres for mouthing off to the referee; it was handing territory to Munster but, in turn, they spurned their half chances.
The sides swapped missed long-range shots 10 minutes into the second half, with Healy shooting wide from half-way following Thomas Larregain's similarly ambitious attempt.
Eventually Munster breached the Castres defence.
After both sides introduced a number of replacements, Munster kept the ball alive playing a penalty advantage.
O’Donoghue looked to be running out of space in the corner but had the power and strength and skill to get the touchdown before being pushed into touch.
Again, the officials took an age to decide that the score was valid, with the on-field decision going the visitors’ way.
But the URC side weren’t to be denied and Healy embellished the score with a smashing conversion from the sideline.
Urdapilleta replied with a penalty before Munster introduced summer-signing Jason Jenkins for his debut.
The versatile South African, who stands 6’8" and weighs in at 19-and-a-half stone, got a warm reception as he replaced compatriot Jean Kleyn.
Healy popped over another penalty with 10 minutes to play but Munster were lucky not to concede from the restart when they went to sleep, allowing a late runner to collect Urdapilleta’s tempting restart.
But Castres, beaten last week at home by Harlequins, did edge in at the end to secure a losing bonus point, number 8 Kevin Kornath barging in for a converted try.
Munster: Patrick Campbell; Andrew Conway, Chris Farrell, Damian de Allende, Keith Earls; Ben Healy, Conor Murray; Dave Kilcoyne, Niall Scannell, John Ryan; Jean Kleyn, Tadhg Beirne; Peter O'Mahony (capt), John Hodnett, Jack O'Donoghue.
Replacements: Diarmuid Barron, Josh Wycherley, Keynan Knox, Jason Jenkins, Jack O’Sullivan, Craig Casey, Jack Crowley, Alex Kendellen.
Castres: Thomas Larregain; Bastien Guillemin, Thomas Combezou, Pierre Aguillon, Filipo Nakosi; Benjamin Urdapilleta, Santiago Arata Perrone; Wayan de Benedittis, Gaetan Barlot, Wilfrid Hounkpatin; Loic Jacquet (capt), Théo Hannoyer; Nick Champion de Crespigny, Simon Meka, Kevin Kornath.
Replacements: Brice Humbert, Julius Nostadt, Antoine Guillamon, Jack Whetton, Mateaki Kafatolu, Rory Kockott, Louis Le Brun, Antoine Zeghdar,
Referee: Matthew Carley (England).