To think that we could have missed out on this. The next two weekends of the traditional back-to-back pre-Christmas games could define European Champions Cup campaigns, and for Irish sides therefore, could possibly shape their seasons. Nobody will win their pools this weekend, much less the cup itself, but they could effectively be eliminated.

Indeed, these uniquely flavoured and pivotal back-to-back affairs are also battles within the war. Akin to football’s two-legged knock-out match-ups, wherein the importance of winning the head-to-head duels over the next two weekends can be critical come the final pool reckoning.

Both Munster and Leinster find themselves in group summit meetings with Clermont and Harlequins, while Ulster face the point of no return at home to the Scarlets. Thomond Park will be a heaving sell-out at 5.30 for the renewal of hostilities with Clermont, who lost on their previous two visits there although they only lost 23-13 here in 2008 after their Canadian firebrand, Jamie Cudmore, had been sent off early on for punching Paul O'Connell before then conceding two late tries.

Coming out of the November Test window, this is the next step down from Test rugby and, accordingly, the front-liners return for some cracking games. Perhaps significantly, the post-November Test window return to European fare generally finds the Irish in good spirits. It is well remembered that Leinster hit something of a high on the corresponding weekend 12 months ago when going to Northampton and winning 40-7.

But, on the same weekend, Munster beat Perpignan 36-8 in Thomond Park, Ulster beat Treviso 48-0 and Connacht went to Toulouse and pulled off one of the great wins in the tournament’s history by 16-14.

Handsomely

Similarly, the year before Connacht and Munster beat Biarritz and Saracens at home, while Ulster won handsomely away to Northampton and Leinster picked up a bonus point in losing 15-12 in Clermont. Twelve months previously there was another treble featuring away wins by Munster and Leinster against Scarlets and Bath respectively.

Part of this may be because the Test front-liners play three rather than four matches and have a week's respite before these Euro games, unlike many of their England and Welsh counterparts. This may conceivably help Leinster and Ulster this weekend, less so Munster given Clermont arrive locked and fully loaded after restoring their rested quartet of French internationals – Sébastien Vahaamahina, Damien Chouly, Camille Lopez and Wesley Fofana – as well as Nick Abendanon, Noa Nakaitaci, the aforementioned Cudmore and the hugely influential Julien Bonnaire.

This is their full-on first-choice team, and rather explodes the hoary old chestnut that French sides are disadvantaged by having to compete more seriously on two fronts. They will travel with ambition and will also be mindful that they can ill afford another defeat in a pool of sharks where the Sharks, ironically, are the only ones who didn’t reach the semi-finals for the last two seasons.

Although Munster lead the pool, they have merely achieved what they were obliged to achieve if they were to advance to the last eight for the 16th time in 17 seasons, and the same applies today as they seek their 100th European win and 24th out of 24 at home to French opposition.

Munster also welcome back five of their expanded Irish squad members – Felis Jones, Simon Zebo, Conor Murray, Paul O'Connell and Peter O'Mahony – but with their Australian centre Andrew Smith ruled out, the 22-year-old Western Province recruit Pat Howard makes only his second start a week after his first lasted only 28 minutes.

Casualty list

Matt O'Connor makes 10 changes in restoring his front-liners for tomorrow's joust with Conor O'Shea's Harlequins, with only Jordi Murphy retained in an all-Test pack due to Dominic Ryan joining Fergus McFadden on their lengthy casual list. Against that, Rob Kearney, Luke Fitzgerald and Ian Madigan have all been seemingly passed fit and named.

As well as retaining Jimmy Gopperth at outhalf, in another signature selection, Darragh Fanning has been retained effectively ahead of the Springboks' back three player Zane Kirchener.

Harlequins have not invested heavily but rather dipped into their academy and have struggled domestically, especially in the last month. But they have performed in Europe and England captain Chris Robshaw will make his 200th appearance for Quins, who also welcome back Mike Brown, Marland Yarde and Joe Marler from international duty.

They do know how to negotiate these games and if they are as bullishly defiant as O’Connor was in his press conference on Thursday, and as resilient and effective as they’ve been on the road under his watch, they could take a stranglehold on this pool with a view to obtaining a home quarter-final.

With the pressure off to some degree as the heavyweight duo of two-time winners Leicester and back-to-back champions Toulon, Ulster and the Scarlets may provide one of the weekend's more entertaining affairs. Ulster are buoyed by the return of Ruan Pienaar and Dan Tuohy.

Although the Challenge Cup offers no passport into the Champions Cup any more, Pat Lam has made only one change for the visit of Bayonne to the Sportsground in recalling Mick Kearney to the secondrow. Four more wins would do very nicely.