As is often the way in elite level football, everything was going perfectly for Manchester City - until it was not.
Having already clinched top spot in their Champions League group thanks to a win against Paris Saint-Germain two weeks ago, City travelled to Germany on Tuesday to face RB Leipzig in their final group game.
Pep Guardiola made seven changes to the side that dominated Watford on Saturday, and City lost 1-2.
In the grand scheme of things the defeat matters little, although Guardiola would have liked to have seen his side maintain their winning rhythm and the likes of Kevin De Bruyne, Phil Foden and Jack Grealish continue their journey back to top form.
What does matter though is that thanks to a nonsensical late red card, Kyle Walker will now miss the first leg of City's last-16 tie, assuming that UEFA do not extend the standard one match suspension.
With less than ten minutes remaining and City a goal down, Walker let out his frustration at a poor personal and team performance by lashing out at the heels of Andre Silva. It was a stupid thing to do, and not something you expect from a player of his experience.
When City's team sheet was published 75 minutes before kick-off in Leipzig, Walker's inclusion was one particular point of intrigue. Why was he not included in Guardiola's wholesale changes?
The England right-back is a near ever-present for City, so it seemed logical that he be rested in what was a dead-rubber for the Premier League champions.
He kept his place though, likely because Guardiola did not want to throw 18-year-old youth teamer CJ Egan-Riley in at the deep end, having given Joao Cancelo the night off.
Regardless, the Catalan coach is now left with a problem he would not have foreseen.
Walker is arguably City's most important defender, particularly when it comes to Champions League games. Domestically City tend to dominate the ball and suffocate their opponents, but in Europe their opponents tend to be more dynamic and willing to counter City.
In the 2-1 defeat of PSG he silenced Kylian Mbappe, and played an instrumental role in City reaching last season's final.
Walker is technically brilliant, but his pace also comes in handy when snuffing out counters and breakaways. The combination of speed and skill is something that his right-back deputy Cancelo lacks.
His absence will be more keenly felt than that of, say, Ruben Dias, John Stones or Aymeric Laporte; all three are capable of filling in. At left-back, too, Oleksandr Zinchenko is a decent replacement for Cancelo.
It's not just in this competition that Walker has enjoyed a brilliant 2021. He was immense away to Chelsea and Liverpool in the league, while he was consistently impressive for England at the European Championship.
City could face some potentially tricky opponents in the last-16, ones they would be expected to beat but who could use Walker's absence to their advantage.
One thing is for certain: Guardiola would not have expected one of his most experience players to be the cause of his latest headache.
Do you think that City will miss Walker in the last-16? Follow our City Is Ours writer Alex Brotherton on Twitter to get involved in the discussion and give us your thoughts in the comments section below.
