So what prompted Gatland to do the unthinkable and leave out a living legend who tour manager Andy Irvine
acclaimed less than 24 hours previously
as "one of the greatest players on the planet for the last 20-30 years"?
Brian O'Driscoll's Lions record
Four (2001, 2005, 2009, 2013)
18 (8 Tests)
Nine (1 in Tests)
Seven times (1 Test)
Two wins, six defeats
The hard-bitten New Zealander is acutely aware of the respect and affection in which O'Driscoll is held, not just in Britain and Ireland, but all over the rugby world.
He knew the seismic tremors that would rumble round it, particularly back home, when people discovered the player most identified with the Lions for the past 12 years would not be granted one final shot at the prize which has eluded him on his three previous tours.
As Ireland's coach, Gatland gave the then 20-year-old O'Driscoll his Test debut back in 1999.
While his subsequent achievements with Wasps, Waikato and Wales are considerable, he also knows his own reputation and rugby legacy are now entwined with Saturday's outcome in Sydney.
If he delivers the Lions' first series victory for 16 years, his selection will be viewed as bold vindication for a man who trusted his instincts and principles in the face of public opprobrium.
Even then, some may not forgive him for ending a stellar Lions career in such brutal fashion.
If the Lions lose a fourth series on the trot, it is hard to envisage Gatland venturing to Ireland, where he cut his coaching teeth, anytime soon.
He could see the storyline the media and public craved - the icon leading the Lions to victory in his final outing in the red jersey.
But it didn't tally with the evidence of his own eyes, or the game-plan he has decided is best suited to beating the Australians.
He has said all along he would pick on form and not reputation. In this instance he has been true to his word, painfully so.
"It wasn't about leadership or picking a captain first," Gatland explained. "It was about picking what we felt was the best team."
Only then did he and his fellow coaches consider who was the best person to lead it.
Alun Wyn Jones,
"one of the first names on the team sheet", was the chosen one.
These days rugby is so structured and analysed, the captain is more of a figurehead, with responsibility delegated around the team rather than invested in one individual.
Jamie Roberts is 'captain' of the Wales defence. Geoff Parling calls the line-out for England and now the Lions. Prop Adam Jones takes a lead at the scrum. Irish fly-half Jonny Sexton is a vocal presence, as all top 10s must be, managing the game plan and calling the plays.
In the returning Roberts at inside centre, Mike Phillips at scrum-half, Richard Hibbard at hooker, Sean O'Brien at open-side and Toby Faletau at number eight, he has summoned five players who will take the fight directly to the Wallabies, ball in hand.
If the Lions can't run round them, they will run through or over them. With Roberts back in harness, it came down to a choice between O'Driscoll and Jonathan Davies for the number 13 shirt.
Ian RobertsonBBC rugby union correspondent
"You just wonder if they got a lucky bag and picked some names out.
"The Lions won the first Test and made eight changes to their squad. The Wallabies won the second and probably won't make any changes. The Lions lose by one point and there are six changes in the team and three guys on the bench who were nowhere near the team for the second Test.
"I was convinced Brian O'Driscoll should have been named as captain. It's catastrophic leaving him out. He's still one of the top centres in world rugby. He's a fantastic guy and has been on four tours and knows it all inside out. It's a massive mistake."
Asked if even he would be on edge about selection after such a devastating loss, there was no sense of immunity to being dropped, but also no indication, outwardly at least, he knew what was coming.
"I won't go chewing my nails off thinking about it," he said. "I will go with the flow like everyone else. Hopefully I will be included, but if I not I will have to deal with that."
It is the first time he has had to, as far as anyone can remember. A fit O'Driscoll has always been in the side for the matches that matter, whether for Leinster, Ireland or the Lions.
How did he respond to this crushing blow? By immediately offering to help out those chosen ahead of him in their preparation, before going off to deliver a coaching clinic for schoolchildren. Such is the measure of the man.
The final indignity came when Gatland was asked whether he was tempted to at least put O'Driscoll on the bench, or whether he had opted for Tuilagi because of his game-changing ability.
"I think you've answered your own question," Gatland replied.
No sense of needing to sweeten the bitter pill. No attempt at obfuscation.
He believes Tuilagi will offer greater impact than a man who has rescued and won umpteen games for Ireland down the years. And on his head be it.
The other, unspoken, reason might be that it will be three weeks since
Roberts damaged his hamstring,
and the demands of his overtly physical role mean he might not last the distance.
The Welshman has been replaced in each of his starts on this tour. Tuilagi offers a like-for-like replacement, and can also cover wing at a push.
From a Lions perspective, it is to be hoped Gatland's gamble - a calculated, if spectacular, one - pays off.
At least then two proud warriors, O'Connell and O'Driscoll, can add a Lions series success to their already overflowing CVs, even if they play no part - on the field at least - in its conclusion.
Any other outcome will leave a bitter taste indeed.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.