With Munster holding a slender 6-3 lead on 50 minutes, the mass brawl that led to a Munster penalty handed the visitors a chance to widen their advantage.
Saracens were potentially on the ropes but JJ Hanrahan’s kick slipped left of the posts and Saracens were energised by the miss, raising their game and gliding through the gears to score tries on 65 and 72 minutes to secure victory.
Munster have it all to do now if they are to get out of this Heineken Champions Cup pool after Saracens climbed into the reckoning with this hard-fought win in London.
Now just a point in front of the defending champions in Pool 4 after four rounds, Johann van Graan’s side have to win in Paris now when they visit frontrunners Racing 92 on January 12 and hope the French club can go to Saracens in round six and do them a favour.
When they needed him most, Billy Vunipola stepped up for Saracens and delivered the game-changing moment, pouncing off the back of a five-metre scrum, the England No. 8 dispatched a wonderful offload out to Sean Maitland to score the opening try that blew the game in the home side’s favour.
It was a pivotal moment backed up by a hard-running, gainline-busting performance out of the top drawer.
Pascal Gauzere had to deal with a mass brawl early in the second half sparked in part by alleged comments made by Munster doctor Jamie Kearns at Saracens hooker Jamie George.
George had appeared to kick the ball at Billy Holland, prompting the incident on the sideline, after which both teams got involved, a number of players in the melee tipping over protective padding onto the athletics track before order was restored by the French official, who awarded a Munster penalty.
There may be further repercussions.
Saracens 5 Munster 12
There was plenty of work for the Munster physios at Allianz Park, starting when captain Peter O’Mahony was ruled out during the warm-up with a groin issue.
Then Tadhg Beirne went down in the 10th minute, leaving on a stretcher with his lower left leg strapped and taking in gas and air.
There was another loss to the pack on 24 minutes when prop John Ryan hobbled off.
Munster have four weeks and three interprovincial Guinness PRO14 fixtures to get through before attentions turn back to Europe and a weekend in Paris at the home of Pool 4-topping Racing 92, La Défense Arena.
M Malins; S Maitland, A Lozowski (D Taylor, ), N Tompkins (Manu Vunipola, ), E Daly; O Farrell - captain, R Wigglesworth (B Spencer, ); Mako Vunipola (R Barrington, ), J George (T Woolstencroft, ), V Koch (J Ibuanokpe, ); J Kpoku (J Wray, ), G Kruis (C Clark, ); N Isiekwe, B Earl, B Vunipola.
M Haley; A Conway (D Goggin, ), C Farrell, R Scannell, K Earls; JJ Hanrahan, C Murray (N McCarthy, ); J Cronin (L O’Connor, ), N Scannell (K O’Byrne, 72), J Ryan (S Archer, 24); J Kleyn (F Wycherley, ), B Holland; T Beirne (T O’Donnell, 10; C Cloete, 72), J O’Donoghue, CJ Stander - captain.
Pascal Gauzere (France)