It was only preseason.

This was the universal footnote applied to Celtic's marquee wins over Manchester City and Chelsea last month.

And it was a fair one. Friendly games mean very little in the grand scheme of things, and there's often an element of going through the motions attached to these stop-gap encounters – no matter the size of the opponent.

Still, the football Celtic displayed against some of English football's biggest stars – not least Raheem Sterling and Erling Haaland – during their tour of the United States was impressive.

Fast, fluid and fierce, indicative of a typical Brendan Rodgers side – at least the one that stormed its way to the 'Double Treble' during the manager's first stint in the Parkhead dugout, or the one that came into its own to clinch the Scottish Premiership and Scottish Cup in the latter stretch of last season.

Those characteristics returned with a bang during Celtic's emphatic 4-0 win over Kilmarnock on flag day last Sunday, with the Ayrshire visitors out on their feet by the full-time whistle.

Having lost 1-0 to Cercle Brugge in their Europa League second round qualifier three days earlier (2-1 on aggregate), Derek McIness' side may have been suffering from a European hangover, but this can't detract from how good Celtic were on the day on home soil.

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Fast, fluid, fierce and, clearly, clinical in front of goal, all underpinned by a solid backline and ever-communicative goalkeeper who looked like he was playing his 100th competitive game for his club, not his first.

Reo Hatate was outstanding. Nicolas Kuhn continues to look reborn. And Matt O'Riley was phenomenal; a mastermind on the ball, and instrumental for his peers all over the pitch when off it.

During the media conference in the days that followed Celtic's thrilling league championship-clinching victory over Kilmarnock at Rugby Park back in May, a jubilant Rodgers quipped about restoring faith in his doubters.

"We're back to zero," he joked when asked about his starting point going into this season being better than the same juncture 12 months earlier. "Hopefully."

It's no secret the Irishman's shock departure for Leicester City in 2019 upset swathes of supporters at the time, and the speed with which goodwill has been recouped among individuals has been variable.

It's safe to say that the vast majority are back behind the manager now – silverware in football tends to have this effect – and if Celtic can continue to extend the fine form they've hauled forward from last term, then that situation can only get better.

In the coming weeks, the Parkhead side face a run of league and League Cup games before their first showdown against Rangers on Sunday, September 1.

By then, the summer transfer window will have closed, and Rodgers will have a clear sense of his squad moving deeper into the year. The newly-formatted Champions League, of course, rolls around a few weeks after that again.

During his media conference last Friday, Rodgers spoke again about making in-roads in Europe, how that might take shape, and what his immediate targets are on the continent this season in particular.

Replicating the performances that outdid Manchester City and Chelsea in competitive terms will be a different challenge entirely. The same applies to mirroring the fast-flowing fare that swept Kilmarnock aside on European club football's biggest stage.

But if Celtic can continue to push their current standards, there's no reason they can't clinch yet another domestic clean sweep, and push their league points tally past a century.

If they do that, who knows what could happen in Europe. In any event, here's hoping there's no footnote required.