The problem for Koeman was not the result, which many had expected before Neymar's injury and a string of encouraging displays in La Liga raised false hope
Ronald Koeman said Barcelona were not ready yet to win the biggest trophies but Paris Saint-Germain showed them on Tuesday how far they have to go.
Kylian Mbappe ran riot at Camp Nou, his hat-trick signing off one of the great invididual performances of the Champions League era and elevating his status to arguably the most dangerous player in the world.
The 22-year-old certainly outshone Lionel Messi, even if snap comparisons are perhaps unfair on the Argentinian, given the mismatch in the teams each had behind them.
This was not a contest between Mbappe and Messi nor a failure on Messi's part that his Barcelona side were overrun, outclassed and ultimately obliterated, a 4-1 defeat putting PSG within sight of the quarter-finals.
Instead, it was a reality check for Koeman's Barca, a brutal assessment of where they are in their recovery and a measuring of the distance still to be covered before they can rejoin Europe's elite.
"I know where we are," said Koeman. "We have to change but we are not at the end of this road, we are halfway through."
The problem for Koeman was not the result, which many had expected before Neymar's injury and a string of encouraging displays in La Liga raised false hope.
If the mauling by Bayern Munich last season was the end of something for Barcelona, a cycle finished and players subsequently discarded, Koeman's project is, in theory, just beginning.
Of those that played on Tuesday, Pedri is 18 and Sergino Dest 20, with Oscar Mingueza, Francisco Trincao and Riqui Puig all 21. Ousmane Dembele and Frenkie De Jong are both 23.
For some of these players, this was the biggest game of their careers and it showed, the usually immaculate Pedri miscontrolling a pass or De Jong stumbling into an opponent instead of gliding by.
And yet what may trouble the players, Messi and perhaps Barcelona's incoming president next month was the extent of the gap, how by the end the full-time whistle felt like a relief.
In an interview with Movistar television afterwards, De Jong struggled to find the words. "I don't know, there is one leg left," he said. "It will be very difficult, we all know that, but we will try."
- 'We have to change' -
"There is always another match," said Koeman. "But after a 1-4 (loss) at home I'm not going to lie to you, there is very little chance."
He had gambled on the fitness of Gerard Pique and then took him off with 12 minutes left, not for injury but preservation. "There was no need to take risks," said Koeman, who was even asked if he had the desire to carry on.
"Fortunately I'm a fairly honest and up-front person," Koeman said. "If I was someone who gets happy when we win and then after a result like today said I had no desire to continue, that would be crazy.
"I know where we are. We have to change but we're not at the end of this road, we're halfway through."
Before kick-off, Koeman had arrived at the stadium gates in his car, with Barcelona fans cheering as they saw him and chanting his name.
Since his appointment last summer, Koeman has stabilised Barca, instilled a sense of purpose and promoted youth. He inherited a mess and has brought direction.
But he will know all that is just a platform, that the weeks between now and June will be what define his season and ultimately whether he stays on as coach.
Barcelona are eight behind Atletico Madrid in the league, trailpoints 2-0 to Sevilla in the semi-final of the Copa del Rey and now have a mountain to climb in Paris.
Some sort of crescendo may be needed. Even if PSG cannot be caught in the second leg, Koeman cannot afford humiliation. Not everything can be put down to transition.
"I am not sad," said Koeman. "It is a path that we are trying to go down and it's not the end of this path yet. It seems I am more realistic than others." Others may not be as patient either.