The Ireland vice-captain pulled his hamstring just before kick-off yesterday.
Ireland concluded their rather mixed Six Nations campaign with a deserved victory over England yesterday, to deprive them of back-to-back Grand Slams.
But Ireland's chances of challenging their visitors looked to be in uncertain territory, with the late withdrawal of Jamie Heaslip due to injury.
There were rumours of a starting player looking to be in doubt just before the game started, and with moments to go before kick-off, it emerged that Heaslip was the casualty.
His absence, combined with the loss of Conor Murray and Rob Kearney, left Ireland without some of their most experienced players for the battle against an England side that was chasing a 19th straight victory.
But the response from the new look Ireland side was emphatic, as Heasplip's replacement Peter O'Mahony came in to give a man-of-the-match performance.
Speaking at the post-match press conference, head coach Joe Schmidt explained the sequence of events that led to Heaslip's injury, and O'Mahony's opportunity.
"It's a bit of a slight on us to be honest, because it's not something that we do. We pick our team and we go out there and prepare them as best we can.
"It was a disruption if anything for us. Dan Leavy got a heck of a shock when I said 'Dan, you need to grab that jersey off Pete (Peter O'Mahony), Pete you grab that off CJ, and CJ you grab that off Jamie cos Jamie's just pulled his upper hamstring and isn't able to play.'"
"It was very late in the warm-up when Jamie stepped aside and the doc had a look at him and tested him inside, I'd say about five minutes before kick-off.
He went on to describe Ireland's second place finish in the Six Nations table as 'important,' but added that there is a lingering frustration that they under-performed earlier in the campaign.