The 2010 championship is a real reference point in the evolution of this Mayo football team. That year they lost their only two championship games to Sligo and Longford, unthinkable now given how consistent they subsequently became.
Ten of the 19 players who started the loss to Longford were still involved more than nine years later when they drew a line under an eventful decade with an All-Ireland semi-final defeat to Dublin.
In between, under Horan initially, then the Pat Holmes/Noel Connelly axis, through Stephen Rochford's three-year term and back to Horan again, no other county, not even Dublin, has such consistency in selection. But the arrival of a new decade has given way to the most significant changes in personnel in 10 years.
Even since the inter-county season resumed just over three weeks ago, the rate of change is even more evident than it was at the start of 2020.
Eight players had made championship debuts by the end of their opening Connacht Championship game against Leitrim. Counties from lower divisions experience such turnovers, but it's unexpected in one that's been in the top flight for the last 23 years.
It's been a theme of Horan's second spell in charge. Any sense that there would be loyalty to that core group of players who served the county so well in his initial four years, that unfinished business with them would cloud everything else, has been misplaced.
That much was probably clear in their opening match of the 2019 league campaign when four players made debuts. By the time they were crowned league champions at the end of March, another six had played for their county for the first time.
This year that pace has quickened. Oisin Mullin, Jordan Flynn, Bryan Walsh, Ryan O'Donoghue and Tommy Conroy (off the bench) made their debuts against Donegal, Padraig O'Hora had his first league start. Since then Eoghan McLaughlin, Paul Towey, Mark Moran, Daniel McBrien and Rory Brickenden have featured for the first time.
In all, Horan has given first Mayo games to 19 newcomers in 26 games since he returned, 20 if O'Hora is included. Some of those had already been on Rochford's radar and featured in Connacht League games, while others were part of a development squad that Rochford ran during his three years.
Consequently, players so synonymous with the last decade - Keith Higgins, Colm Boyle, Tom Parsons, Donal Vaughan and Seamus O'Shea - have yet to play in any of the four games since the resumption, though injuries may be a factor. Higgins and Boyle made the bench for the win over Roscommon last weekend.
Still, it's quite clear sentiment isn't shaping selection. Horan has thrown the net far and wide to usher in a new era - using 49 different players over the last two seasons - just as he suggested he would do prior to succeeding Rochford last September when he was given a four-year term.
Back then he identified "10 to 12" players he felt could make a difference, expressing real excitement about the county's future.
It wasn't a view shared by everyone but in Mullin, McLaughlin, Matthew Ruane, Ryan O'Donoghue and Conroy especially, there's a quintet offering real hope for the future.
Former Mayo footballer Liam MacHale feels his old playing colleague Horan had "no choice" but to implement such change.
"It's great that he is playing some of the younger players but there is part of this as well that he has no choice, some of the lads are gone, the legs are gone, we all know what that's like. They're not as good as they were. They'll probably be gone next year. Two or three would probably be enforced, two or three, it's giving guys a run."
What MacHale likes most about the new breed is how 'condition-ready' they are for inter-county football.
"Those who have come in are all conditioned, all athletic, all strong. They have that attitude and understand what mindset and effort it takes to play at this level.
"What they have to do is maintain what they have, maybe get a little bit stronger. Mullin at corner-back looks like he is as strong as a bull, McLaughlin on the wing is physical and athletic, the reincarnation of Colm Boyle with a left foot. Now they have to figure out how to play the game at this level.
"McLaughlin just needs to slow down and figure out that he doesn't have to play in fifth gear all the time, there's fourth gear. And that's going to take a bit of time.
"The place to figure this out is on the playing field, not sitting on the bench."
Conroy is the kind of player MacHale feels Mayo haven't had for years. "When you don't have that pace in the full-forward line, full-back lines defend you differently.
"A defender afraid of that pace will always be cautious, always stay slightly behind," said MacHale, who likes the balance being struck with the power and vision of Aidan O'Shea at full-forward and the clinical nature of Cillian O'Connor since he came back.
O'Connor looks to have energy back in his legs after sitting out the early part of the last two years to allow injuries to rehabilitate.
O'Shea, on the other hand, is the one virtual constant in all of Horan's 'second coming' teams.
From the 26 games played through the 2019 and 2020 leagues and championships so far, he has only been off the field for 50 minutes - the first half of the Donegal league game on the opening night in January and the last 15 minutes of the second round against Dublin.
In 2019, he played every minute of eight league and nine championship games.
MacHale has always advocated O'Shea being positioned at full-forward and feels they can get even more out of him.
"They are not using him as well as I would like, especially with the mark. I'd like to see them working on some diagonal balls when the defence shifts and kick some long ones to the far post the way Kerry used to use (Kieran) Donaghy. It's very difficult to defend against," reasoned MacHale.
"I'd have always said play him like (Michael) Murphy. When you have the breeze or are on top, play him inside. Against the breeze or if you are struggling to get on to possession or hold on to possession around the middle, bring him out. He is skilful enough to do both. He doesn't have to be there all the time."
With David Clarke, Chris Barrett, Lee Keegan and Kevin McLoughlin they are providing that bedrock of experience, together with Paddy Durcan and Diarmuid O'Connor, that allows the transition to continue while creating the impression that it's somehow business as usual.
MacHale feels the transitory nature of the team offer cover for a real go at a championship this year because the pressure is, to some degree, off.
"Mayo haven't won a Connacht title for five years, it has to be very important for these players. Win that and for the supporters, the pressure is off," he suggested.
"The young lads will be delighted with that win and everything feels like a bonus after."
But within the squad there will also be a realisation that with Kerry gone, it's a situation to exploit.
"Mayo are in a right good position, especially with Galway sitting on their hands for the last three weeks. I'd say James knows a bit more about his team than Padraic (Joyce) knows about his.
"As far as I am concerned, Mayo are still a long way off Dublin. But beat Galway and the team could get into an All-Ireland final.
"Then it's league football no matter what way you put it. All-Ireland semi-finals and finals won't be anything like August and September. Get a win on Sunday and crazy stuff can happen."