The stars were out for Scotland's titantic Euro 2020 tussle with England on Friday.
International football's oldest fixture didn't deliver the goals but nonetheless proved a gripping spectacle.
And watching on from the stand were a pair of interested Celtic observers.
Here's 5 potential takeaways from their visit to London.
CalMac on top form
Callum McGregor served a timely reminder of his quality with a hugely assured and controlled display in Scotland's midfield.
Billy Gilmour's exciting breakthrough performance understandably snatched the bulk of the plaudits but he was partnered by a player who can clearly still operate at the top level.
McGregor has looked jaded and inhibited for Celtic but perhaps it's no accident two of his best performances over the past year have come in a Scotland jersey. He was exceptional in Belgrade for the play-off final and pivotal in London.
The 28-year-old will be a much-needed constant in a summer of flux and finding the right system to help him flourish again will be key for Postecoglou.
Next captain decision?
McGregor's display has probably confirmed a decision most expected Postecoglou to take anyway.
Scott Brown's departure leaves the armband up for grabs but if there was ever any doubt over who was next in line, it was surely dispelled in the English capital.
Again, with players likely to come and go and considerable numbers, giving McGregor the platform to provide valuable continuity with his leadership would be a shrewd move for Postecoglou.
O'Donnell's quality reminder
Postecoglou's eye for detail likely means he's already clued up on Scottish football and what it will hold for him.
But this was the first opportunity he's had to witness first-hand the quality the Premiership can offer outwith Celtic and Rangers.
Stephen O'Donnell soaked up heavy criticism for his shaky Czech Republic display and bounced back to match up with the multi-millionaires of the Premier League.
The Motherwell defender was outstanding on the biggest stage and Postecoglou will have noted his performance.
Stars on the bench
If departed Wembley with one regret, it's that Postecoglou will have hoped for a glimpse of few more of his new Celtic charges.
McGregor was the only Parkhead star to feature, with Ryan Christie, Greg Taylor and James Forrest confined to the bench throughout, while David Turnbull was cut from the squad entirely.
Of course, the new manager will welcome the players to Lennoxtown before long but a chance to see how more of them coped on such a big occasion certainly wouldn't have gone amiss.
Tuesday's do-or-die showdown with Croatia could offer another chance but Steve Clarke seems unlikely to tinker his starting side too much, if at all.
A taste for the big occasion
Wembley wasn't quite at full capacity but the excitement and grandeur of the occasion are exactly what Postecoglou longs to experience at Celtic.
Last week, he spoke of bringing Champions League "magic" back to Parkhead and taking in one of Europe's premier stadia will have only heightened that ambition.
The clock is ticking down to Celtic's second round qualifier against Midtjylland and the club face a treacherous path to the group stage.
But should the manager pull off the unlikely and the return of supporters to Celtic Park materialise in the coming months, he'll be soaking up an atmosphere few venues can claim to match.
Source : https://www.msn.com/en-gb/sports/football/ange-postecoglous-5-celtic-takeaways-from-wembley-trip-as-new-managers-first-big-decision-made-clear/ar-AALgpiU682