The one English exception, Saracens, will be knocked off their perch should the Ospreys prove more of a handful in Swansea on Friday night than at Wembley last Saturday evening when they went close to saving the tie of the round.
The competitive nature of the tournament is such that only four of the 24 contenders have got this far without losing. Three - Cardiff Blues, Munster and Leinster - are from the RaboDirect, one - Toulouse - from across the Channel.
Of the 13 in varying stages of contention, almost as many are to be found amongst the ranks of the RaboDirect as the Top 14 and Aviva Premiership combined -- Scarlets (10 pts), Edinburgh (9), Ulster (9), Glasgow (8), Ospreys (7), Treviso (6). The Ospreys are not alone in appreciating that this weekend will determine whether they will still be in realistic pursuit of a quarter-final place come the New Year.
Edinburgh have been there just once despite their status as perennial Heineken Cup competitors since the beginning in 1996. If they are to get there again after seven successive eliminations at the pool stage they must avenge last week's setback at Cardiff and beat the Blues at Murrayfield on Friday night.
After scoring four tries there in the course of a runaway win on the opening weekend of the domestic season in September, the Blues under the guiding boot of Scotland's Dan Parks will be less than terrified at the prospect of making it four European wins out of four. Edinburgh believe that this time Murrayfield will give them a canvas broad enough to follow their epic win there over Racing Metro last month.
'The size of the pitch allows us to play an expansive game which suits us down to the ground,' Edinburgh captain Greig Laidlaw said. 'It's a big pitch and that helps us get our free-flowing type of game going.'
A home win would put them on track for a winner-take-all finale during the last round of the pool stage back at Murrayfield against London Irish, provided the Exiles continue to catch up for lost ground by completing the double over Racing at the Madejski on Saturday.
The Parisians are not even running let alone racing after losing all three ties, a record which puts them down among the early casualties although they can claim to be in reasonable company. Northampton, beaten finalists last season when Leinster required one of the great comebacks to regain their crown as champions of Europe, are also out, surprisingly so given that it took something extraordinary even by their standards for Munster to deny them a win at Thomond Park on the opening weekend. The margins between success and failure can never have been that fine.
Glasgow's famous win in the same city three seasons ago will be at the back of their minds en route to Saturday's return in another part of southern France. Montpellier, denied victory over Leinster by Jonny Sexton's last-kick penalty in the opening round, are still without a win after successive trips to the west of England and west of Scotland.
Instead of seizing complete control of their pool, the Scarlets' failure to see Munster off last week leaves them the highest ranked of the second-placed teams. If they are still there at the end of the pool stage they will reach the last eight as one of the top two runners-up.
The west Walians are aiming for something more uplifting. In crossing the Irish Sea for the Sunday kick-off, the west Walians will be flying in the face of history because if they are to avenge last Saturday's narrow home defeat, they will have to do something which has never been done before.
No Welsh team has won a Heineken Cup tie in Limerick and it hasn't been for the want of trying, both before and after the old clubs were superseded by the four regions. Pontypridd, Newport and Bridgend lost there in the early days since when they have been joined by region after region.
The Scarlets, beaten 22-13 on their last European visit four seasons back, will go for broke, just as they did at Northampton in seizing all five points. Trying to overwhelm Munster in the same manner will require something truly amazing, not that anyone ought to forget that the Scarlets have done that sort of thing before. Six years ago at Toulouse, of all places, they turned an apparently lost cause into a sensational away win, wiping out a huge deficit to win 41-34.
Matthew Rees could do worse than recall that match as an example of what can be done, not that the former Wales captain is short of belief. 'While I'm sure nobody will give us a hope in hell of winning at Thomond Park, we have proved in the past that we can win big away games in the Heineken Cup,' he said. 'We just have to go with the right attitude and commitment and show them what we can do.
'We've got to block out the crowd and just play what's in front of us. We only need to look back at the Northampton game. We went up there, chucked everything at them and came away with a great result. There's no reason why we can't do that again.'
Saying it is one thing, doing it another matter entirely, not that Munster will take anything for granted after the trauma of failing to survive the pool stage last season.
How they stand:
Pool 1:
Top - Munster. In contention - Scarlets, Castres. Out - Northampton.
Pool 2:
Top -- Cardiff Blues. In contention - Edinburgh, London Irish. Out - Racing Metro.
Pool 3:
Top - Leinster. In contention: Glasgow, Bath. Out - Montpellier.
Pool 4:
Top - Clermont Auvergne. In contention - Ulster, Leicester. Out - Aironi.
Pool 5:
Top - Saracens. In contention - Ospreys, Biarritz, Treviso.
Pool 6:
Top - Toulouse. In contention - Harlequins, Gloucester. Out - Connacht.
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