O'Brien has been total colossus but time's up
I heard Jasper Carrott on the radio during the week - one of my favourite lines from that particular comedian was about age and drugs.
"There are drug tests everywhere in sports now. Even the old guys who play crown green bowls have been giving urine samples. They don't mean to…"
Age is relative and it can be difficult to decide what age you really are in the game of rugby union.
Last Saturday in the Aviva a lot of players on both sides showed their age and it was telling - some have crossed the Rubicon and some might never even get there.
Seán O'Brien has been a sensational player for Leinster, Ireland and the Lions but I think his time has come.
After a poor Six Nations, a redemptive quarter-final against Ulster - playing against two former Leinster back-rows who weren't good enough to play continuous rugby with the Blues - was just what the doctor ordered.
O'Brien turned 32 last February but played like a 42-year-old and was withdrawn on 52 minutes after another ineffectual performance.
It was interesting to note that when Dan Leavy suffered his season-ending knee injury Leinster chose to send on Mick Kearney and move the 34-year-old Scott Fardy into the back-row, leaving it long on blindsides and lacking any true opensides.
When veterans play at this level there is no substitute for their knowledge and experience - a lifetime where impulse turns to instinct and you automatically know the right thing to do at the right time.
O'Brien knows exactly what to do, he knows how to do it and he knows when to do it, and his skill-set is such that he can do it in his sleep, but at 32 years of age, after spending too many seasons in the tumble dryer of injury, O'Brien is shot.
Physically, at this stage, I don't think his body is up to the carnage and the rough and tumble of the physical exchanges of the kind of match we saw last Saturday.
What you are playing O'Brien for is his voice, his leadership, his command of the game and his superior rugby knowledge. To have somebody of his calibre in training, on the team bus, in the dressing-room and out on the pitch is comforting, but that comfort comes at too high a cost and the bare bones of Seán O'Brien's under-performance last Saturday does not merit reselection.
He had nine runs for five metres at the Aviva - that's heading into Quinn Roux territory.
His 12 tackles fell way behind that of Jack Conan (16) and Rhys Ruddock (17), and a long way down on the incredible 27 that James Ryan had. Across the other side of the scrum Nick Timoney had 21, Jordi Murphy 22 and Marcell Coetzee 16.
Coetzee provided the sort of ballast and dynamic ball-carrying that you would normally expect from O'Brien.
This is one of many selection issues that Leo Cullen will have to address because if changes are not made for the Toulouse game, they will lose that semi-final on April 21.
The key man in a chess game of selections will be Nick McCarthy.
The Leinster third-choice scrum-half, who is heading south to be Conor Murray's understudy next season, must be given game time against Benetton and Glasgow in the lead-up to the semi-final, as must Luke McGrath.
McGrath had a decent game on Saturday and if everything, understandably, was not quite up to speed because of his injuries, at least his passing was quick and accurate, and his box-kicking was decent.
You have to look at where Leinster are vulnerable and what Toulouse can do, and there is no more glaring weakness that any video coach could find than at number 14 on the Leinster team sheet.
Ulster went after Adam Byrne, literally, from the off, and had a great opportunity in the third minute off a scrum near the Leinster 22 on the 15-metre line.
Ulster's scrum was solid and John Cooney had enough time to do exactly what he wanted.
With that sort of a blindside, if you can get Jacob Stockdale running off a flat pass one-on-one against anybody in the world then you have a chance. Dan McFarland and his team could not believe their luck that they got to try out this move this early in the game.
Stockdale pretended that he was looking to join in an openside back-line move but wandered around as Cooney pivoted back to the blindside and fed Stockdale flat and at pace.
Byrne missed the straight-up tackle and Stockdale was barely pushed into touch a metre from the line by a posse of four Leinster players. Fifty minutes later came the match-defining moment. Stockdale showed devastating pace to the corner before he dropped the ball but the question nobody has asked was how he managed to get outside.
Once again Byrne came in when he had no business to and again he got caught ball-watching and Stockdale was afforded space on the outside. Jordan Larmour did not cover himself in glory either as he missed a straight-up tackle on the Ulster flyer.
But the opportunity was spurned and Leinster survived.
The bottom line here is that James Lowe must start against Toulouse.
He must start because in the two matches against Toulouse in the pool stages, Lowe was the touch paper for everything constructive that Leinster did. He will cause serious damage to Toulouse, who are really dangerous with the ball but will once again struggle to contain the Kiwi in blue when he has the ball.
I think Larmour will have to be moved to the wing. Some of the mistakes he made will prove too costly if Toulouse can take advantage of them. That is why Rob Kearney will start at full-back, with Larmour and Lowe on the wings.
I do not think it is prudent to pick Robbie Henshaw at inside centre even though his abilities for this match appear to require his selection. The Leinster centre's powers of recovery are remarkable, but also his propensity for getting re-injured is even more so.
Rory O'Loughlin did reasonably well for the time that he was on the pitch and he may be a better bet than Henshaw, who just could not be match-fit in time.
One of the glaring deficiencies in Leinster's back-line last Saturday was how poor their passing was.
The quality and crispness of the Ulster back-line passing, even with Stuart McCloskey at No.12, blew Leinster away.
The two best half-backs operating in Ireland at this moment in time are John Cooney and Billy Burns. Burns' passing was smooth, he got his back-line going and his game management surprised me with its quality.
Johnny Sexton may not have had his best Six Nations, but for this Leinster back-line to maximise their potential they need Sexton, because Leinster will need to score a hatful of tries to beat Toulouse.
If Devin Toner is fit that will be great news, if not they will have to play Fardy in the second-row and bring in Max Deegan at openside.
There are difficult decisions to be made, but if Leinster want to advance to yet another final then that will have to be done.