The Leinster and Ireland full back was laid low with mumps in the last part of the season and lost a full stone in weight as he spent almost two weeks in bed.
It meant him missing Leinster's historic Heineken Cup semi-final clash with Munster at Croke Park, although he did recover in time to pick up a winner's medal in the finl.
But the prize on offer now is even bigger as the clock counts down to selection for the first Test against the Springboks in Durban next weekend.
To date, Ospreys and Wales full back Lee Byrne has started in three of the four tour games and was a try scorer again in the 39-3 triumph over the Sharks on Wednesday night.
Now it is Kearney's turn to show what he can do and he intends to make it difficult for Lions head coach Ian McGeechan to pick between his two No 15s
"I know that a solid game isn't going to be good enough to get me into the Test side so I'll be looking to show what I can do," said Kearney.
"There is a lot riding on this game and we all know there are Test places up for grabs.
This is only my second game on tour - I'd have liked to have had one or two more, so it is all or nothing now.
"It's all going to come down to Lee's performance on Wednesday against the Sharks and mine at Newlands. After that they'll weigh up the two options.
"All I want to do is go out and have a strong game to put myself in the strongest possible position. I just have to dig deep"