O'Brien has had his own injury problems but has shaken off the calf problem that has delayed his first appearance on tour. His recovery is timely for the Irishman is just the sort of thundering presence that the Lions need, especially with Matt Todd in the Crusaders' No. 7 jersey.
Even though there was more clatter to their play in Auckland on Wednesday the Lions have yet to set the opposition back on their heels. O'Brien has the potential to do just that, a different sort of player than Warburton, and more multi-faceted in that he is at ease in any position across the back row.
"That makes the odds a bit better in my favour, doesn't it?" said O'Brien with a chuckle. "Any opportunity I'm given I'm going to do what I can do. 'Warby' is captain but, as you've seen a few years ago, anything can happen and it's about putting your best foot forward and making sure your hand is up for selection. That's what I want to do.
"I want to be in the Test side, as does every other player. I'm not going to be banking on Sam being injured or not, I'm going to try and play my game and hopefully that's good enough to be selected."
O'Brien did just that in Australia four years ago, coming on in the second Test and starting in the decisive third which Warburton missed through injury.
At his best, there are few more powerful and destructive players and there is a growing sense that the Lions need to assert themselves physically rather than try to go toe to toe or ¬offload to offload with the Kiwis, who are masters of the counter-attacking game.
O'Brien has the necessary presence and has already made an impression.
"Sean is class and a proven performer, as we saw on the last tour," said Rowntree. "He brings many attributes to the game on both sides of the ball. I like his energy in the group. He's an impressive character."
The Lions need those sort of individuals to come to the fore over the next fortnight, whether Warburton is fit or not.
The management have already run the rule over possible replacements as captain were Warburton not to pass muster, from today's captain, Alun Wyn Jones, to the likes of Rory Best or even Owen Farrell, a natural leader in all that he does.
Rowntree added: "There are a few candidates that we spoke about initially when we considered tour captaincy so there are some other guys we can call upon."
O'Brien has played his part in that regard as one of the senior figures and will also be able to draw on his experiences of playing the All Blacks even if he did miss Ireland's victory against them in Chicago.
"There are younger guys out here and you just pick them up straight away [after a loss] and brush them off," said O'Brien.
"There's no point dwelling or sulking over something at this stage. We don't have the time for that, it's pretty hectic and you have to move on very quickly."
