It's a must-win for the Pool 3 back-markers from Wales, but it's going to be mighty tough for them in Dublin.
For a side without Brian O'Driscoll and Denis Hickie, Leinster performed admirably against Sale Sharks on Sunday. Things went wrong at the start, the Sharks started well and the Lions lost key players, in Eric Miller and Victor Costello, but didn't panic and their replacements really performed.
It was a very efficient display from Gary Ella's side and they will expect to harvest a win against the Blues in front of what could be the biggest crowd yet seen for a Heineken Cup Pool game.
Gary Ella: Leinster coach
Cardiff at last got a win, and some luck, against Biarritz but a very bad miss from Dimitri Yachvili - he would normally expect to slot that conversion.
The Blues could have gone very close in this pool...They could have beaten Sale and Leinster, and didn't play that badly in Biarritz.
Prediction: Leinster have got to go for it as they need to finish as one of the top four pool winners to earn a home quarter-final. I expect them to ease through this one. I have always fancied that this could be Ireland's season again in the Heineken Cup and in Leinster and Munster they have two very strong candidates.
Meanwhile, Celtic Warriors are another Welsh side who simply have to win this week when they tangle with last year's runners-up Perpignan at the Brewery Field.
Failure to do so would effectively put Pool 6 down to a showdown between Perpignan and London Wasps in the deep south of France the following week, but I would not rule out the Warriors.
The Warriors got a bonus point in Perpignan, while the Frenchmen were not convincing against bottom club Calvisano over the last few weekends. Lynn Howells' side have home advantage, but will those two titanic tussles with Wasps have taken too much out of them? I don't think so.
Many were dismissive of the Warriors in this group, but they have good players and I believe that feeling came from people outside of Wales being unaware of what this new region was. They are a good organisation and have seen off talk of a merger with Cardiff. The penny should have dropped by now.
Prediction: Friday night at home, with another big Brewery Field crowd, places the Warriors as my favourites. It would make it an interesting pool and, with three closely matched sides guaranteeing that winning the section will be the only certain way of qualifying.
Who would have thought that Edinburgh Rugby would be the only side in the Heineken Cup with a 100 percent record after four matches? Not many, and I would like to see someone claim it if they did!!
Edinburgh: On verge of history
It's great news for Edinburgh, the competition and Scottish rugby, and they are on course for a final day showdown with Toulouse - who are just two points behind the Scots due to picking up more bonus points.
However, victory over Neath-Swansea Ospreys would be an historic moment as it would ease Edinburgh on to five wins from five and guarantee a first-ever quarter-final place for a Scottish side.
Prediction: I think Edinburgh will do it, although there will be a spirited response from the Ospreys. It was a bold move to convert Welsh professional rugby into regions, will take time for some of them to bed down and, as the Ospreys have found out, the transition is easier if you are winning.
Llanelli Scarlets have got to go hunting for a bonus point against The Borders. Northampton Saints failed to do that against the Scots at home, which could prove costly, so Gareth Jenkins' men have got to make this a five-point game as they must go to Franklin's Gardens in round six.
That fixture will not be easy with Northampton, whatever happens in Agen on Friday, still in it. The Scarlets secured a bonus point for a narrow defeat at Stade Armandie, which was very significant as this pool rolls inexorably towards a tight finish with every point vital.
Prediction: What I will be looking for from this match is if the Scarlets have the confidence to cut loose. They have been doing well up front but have talent behind the scrum which has not been unleashed. If they go for the jugular they should get five points.
The big question in Pool 4 this weekend...Can Northampton Saints become only the second visiting side in 54 matches to be victorious at the Agen `bear-pit`? Find out with our delayed coverage from southern France.
Agen-Northampton: Tough for Saints
It will be so difficult, if they return with something then it will set up a massive finale with Llanelli but even a bonus point defeat could leave the door open for Agen, who are due to play the tailed off Borders at the same time.
Pool 4 is another group where winning the section is the only guarantee of progression and Saints will be hoping for better conditions (although they could hardly be worse) than Llanelli faced last week. The Scarlets should have taken the boat.
Prediction: I have to go with the form an pick Agen to continue their enviable home record. Saints coach Wayne Smith says no-one expects them to get anything at Stade Armandie...He's right, and if they pull it off it will be Northampton's greatest performance under his stewardship