It is not a reunion that any Bayern Munich fan will be relishing when Robert Lewandowski returns to the Allianz Arena on Tuesday.

Just a few months ago they were cheering goal after goal the Poland striker was scoring for their team, something he had done with tremendous consistency ever since arriving from Borussia Dortmund.

Then came the news none of them will have wanted to hear, that Lewandowski wanted a new challenge.

After a surprising amount of unpleasantness between player and club during the transfer window, the 34-year-old got his wish and made the move to Barcelona for a reported fee of €50million.

With a sense of inevitability as the balls were opened by former Barca midfielder Yaya Toure during the draw for the group stage of the Champions League, who should Bayern be joined by in Group C along with Inter and Viktoria Plzen? Of course, Barcelona.

Ahead of Lewandowski lining up on the opposing side in Munich, Stats Perform has taken a look at what he achieved at Bayern, and how both parties have adapted in the early stages of the new campaign.

The man they called "Lewan-goal-ski" (well, Thomas Muller did)

Of course, Bayern could not feel too bad about having their main goalscorer taken from them, considering that is exactly what they did to chief Bundesliga rivals Dortmund when they signed Lewandowski on a free transfer in 2014.

Inevitably, it turned out to be a key move as BVB fell away after struggling to replace him, while Lewandowski went on to score an exceptional number of goals at his new home.

Overall, he scored 344 goals and recorded 57 assists in 375 appearances for Bayern, and in the 2020-21 campaign, he broke Gerd Muller's long-standing Bundesliga record by scoring 41 times in a single season, while his 43 league goals in 2021 serve as the record for a calendar year in Germany's top tier.

Last season, Lewandowski scored 50 goals across all competitions, the most across Europe's top five leagues, as Bayern lifted their 10th Bundesliga title in a row.

In all, he won eight league titles, three DFB-Pokal's, as well as a Champions League, UEFA Super Cup and FIFA Club World Cup.

After that, it was 'Lewan-gone-ski', as team-mate Thomas Muller may well have called him after his departure, before holding far too long for applause.

What is 11 minus a nine?

With arguably the best number nine in the game gone, Bayern head coach Julian Nagelsmann wanted to evolve his team, making them less reliant on one figure for so many goals and spreading the responsibility.

Sadio Mane joined from Liverpool as the de facto replacement, and before the DFL-Super Cup win against RB Leipzig, Nagelsmann admitted: "When we agreed to Barcelona's offer [for Lewandowski], it was planned that we might not sign anyone else for this position."

The theory was that the likes of Mane, Leroy Sane, Kingsley Coman and Serge Gnabry, fresh from signing a new contract, could increase their output in front of goal, while the ever reliable Muller and the increasingly promising Jamal Musiala would also be potent sources.

Things certainly started well enough with a 5-3 win against Leipzig, followed by a 6-1 thrashing of Eintracht Frankfurt in their opening Bundesliga game.

After a 2-0 victory at home to Wolfsburg, Bayern put seven past Bochum and talk of a lack of goals could not have been further from anyone's lips.

However, three draws in a row in the league against Borussia Monchengladbach, Union Berlin and Stuttgart have followed, which has seen Union emerge as the surprise Bundesliga leaders after six games.

In those games, Bayern have scored a total of 19 goals, with 10 different scorers, registering 12 points in the Bundesliga.

By comparison, in their first six league games last season when Lewandowski was still front and centre for them, they scored 23 goals, though with only seven different scorers, and the Pole providing seven goals of his own, and had 16 points after five wins and just one draw.

In their opening Champions League game, though, Nagelsmann's men put in a terrific performance as they beat Inter 2-0 at San Siro, with their second goal in particular showcasing the sort of passing and moving around the box that feels more possible when you don't have an orthodox number nine as the obvious target.

Lewandowski picks up in Spain where he left off in Germany

Though it took a bit of, shall we say, moving things around so Barcelona could register their new star striker, along with a number of other signings in the transfer window, there has been very little adaptation needed for Lewandowski in LaLiga.

He has already scored six goals in his first five league games, making him the fastest player to reach that figure in the competition in the 21st century.

The forward also has two assists, which makes him the joint-fastest to have been involved in eight goals in the 21st century, alongside former Barca players Rafael van der Vaart in 2008 and Cesc Fabregas in 2011.

Lewandowski also added three more goals to his impressive total in the Champions League with a hat-trick in Barca's 5-1 win against Viktoria Plzen at Camp Nou last week.

That made him outright third in the competition's all-time leading scorers with 89 goals in 107 appearances, behind only Lionel Messi (125 goals in 157 games) and Cristiano Ronaldo (140 goals in 183 games).

Of course, Lewandowski has recent history of this fixture, playing for Bayern as they beat Barca 3-0 home and away in last season's Champions League group stage, scoring twice in the first game in Spain.

It played a big part in the Catalan giants being dumped out of the competition at that stage for the first time in over 20 years, but with Lewandowski on board, Xavi's side will be hoping he can fire them to the round of 16 and beyond, just as he did so regularly for Bayern.

Lewandowski has already started to do so with his treble against Plzen, but will he be able to make an impact again when he faces his former club, or will Bayern be able to prove they have started to move on without him?