Mueller instantly blossomed under Flick, grabbing 12 goals in 47 games in all competitions and 25 assists, a record 21 in the Bundesliga, as Bayern came back from sixth place midway through the campaign to win the league for a record-extending eighth consecutive time
Bayern Munich's Thomas Mueller scored twice in their 8-2 demolition of Barcelona in the Champions League quarter-finals on Friday, shining again on the biggest stage of club football after a rollercoaster season.
Only a few months ago the 2014 World Cup winner could not have imagined being centre stage of a European classic, having been overlooked by former coach Niko Kovac.
Dropped in 2019 from the national team, the attacking midfielder spent much of the season start brooding on the substitutes' bench as Bayern struggled under Kovac.
"I could not understand it then nor can I now that I had to sit on the bench for six consecutive matches," Mueller, who came through the Bayern youth teams, had said after Kovac was sacked in November and replaced by Hansi Flick.
"I always had the feeling that I can help the team. This situation with me at the age of 30 was not my idea of a permanent situation."
Mueller instantly blossomed under Flick, grabbing 12 goals in 47 games in all competitions and 25 assists -- a record 21 in the Bundesliga -- as Bayern came back from sixth place midway through the campaign to win the league for a record-extending eighth consecutive time.
They also won the German Cup and on Friday lived up to their favourites' tag for a treble with the Champions League title as Mueller became Germany's record Champions League player with his 113th appearance.
He added two more goals on Friday, scoring their opener in the fourth minute and grabbing their fourth in the 31st to put them on the road to one of the biggest wins in the history of the competition.
"We had so much fun today. Of course, Barcelona are special, so we had to be even more aggressive and come back again and again. When they press and dribble, it's not a problem. When we get the ball, we have so much talent in attack," Mueller said.
"You can imagine we feel good after this match but we have to be quiet and regenerate. Tomorrow morning, we will wake up and answer messages on our phone from home and the whole world. Then we have to focus on the next game. We are here to win the final."
Mueller became the only player in the Champions League to have scored six times against Barcelona.
"Thomas Mueller is unique. He cannot be copied," Flick had said three days ago.
On Friday, Mueller proved him right.