The most reliable trophy-machine in European football over the last 12 months roars into Paris on Tuesday. But you don’t need to listen too hard to hear the engine putting, some of the gear changes clanking awkwardly. Bayern Munich’s smooth defence of the mass of trophies they collected from 2019-20 has hit some awkward bumps. The most damaging, potentially, is the deficit they take into the second leg of their Champions League quarter-final with Paris Saint-Germain, <a href="https://www.thenationalnews.com/sport/kylian-mbappe-and-psg-ride-their-luck-in-thrilling-champions-league-win-at-holders-bayern-1.1199290">who won 3-2 in a snowy Munich last week</a>, a handsome halfway lead in the tie, given the three away goals. Bayern face a tricky equation: They must win 2-0, 3-1, 4-2 or better to affect a turnaround in 90 minutes. And they must achieve that margin of victory without two of their most efficient goalscorers. Leading marksman Robert Lewandowski is out,<a href="https://www.thenationalnews.com/sport/football/bayern-munich-face-up-to-robert-lewandowski-injury-blow-as-borussia-dortmund-brace-themselves-for-erling-braut-haaland-exit-1.1195641"> still recovering from the knee injury</a> he picked up while <a href="https://www.thenationalnews.com/sport/football/robert-lewandowski-ruled-out-of-poland-world-cup-qualifier-against-england-1.1193121">on international duty with Poland last month</a>. Serge Gnabry is in self-isolation following a positive Covid-19 test. Leon Goretzka, a regular supplier of goals from midfield, also has a muscle strain that puts his full participation in doubt, and Kingsley Coman, the winger and scorer of the goal that <a href="https://www.thenationalnews.com/sport/football/champions-league-final-kingsley-coman-scores-winner-as-bayern-munich-beat-paris-saint-germain-1.1067471">delivered Bayern the last European Cup final at PSG's expense</a>, suffered a knock at the weekend. Coman's best understudy, Douglas Costa, was already on the injury list. And then there's the background noise. At Bayern, a club of vast ambition, internal fault lines between the levels of management tend to be quickly amplified, not least when standards slip on the field, as they did last week in the Champions League and again in <a href="https://www.thenationalnews.com/sport/football/bayern-munich-held-psg-run-riot-ahead-of-champions-league-second-leg-showdown-1.1200896">Saturday's 1-1 draw with Union Berlin</a>. Tensions between Hansi Flick – the coach promoted from his assistant's role in November 2019 to lead a run that delivered a <a href="https://www.thenationalnews.com/sport/football/bayern-munich-lift-bundesliga-trophy-after-4-0-thrashing-of-wolfsburg-1.1040119">Bundesliga title</a>, a <a href="https://www.thenationalnews.com/sport/football/51-goals-for-robert-lewandowski-and-another-double-for-bayern-munich-in-pictures-1.1044290">German Cup</a>, a Champions League, domestic and <a href="https://www.thenationalnews.com/sport/football/more-than-15-000-watch-bayern-munich-lift-uefa-super-cup-in-budapest-in-pictures-1.1083175">European Super Cups</a> and the <a href="https://www.thenationalnews.com/sport/football/bayern-munich-beat-tigres-and-win-club-world-cup-1.1164712">Club World Cup</a> – and Hasan Salihamidzic, the sporting director, have overshadowed the lead up to the Paris expedition. Both Flick and Salihamidzic are former Bayern players. They have held different positions on transfer priorities over the past three transfer windows. “The squad we had last year was stronger,” said Flick, aware it would sound like a criticism of the club’s strategy, ahead of the trip to Paris. In fact, the personnel changes since Bayern beat PSG in last August’s final are relatively few – Thiago left for Liverpool, Philippe Coutinho and Ivan Perisic returned to their parent clubs after loans at Bayern – but Flick had pushed for the purchase of established but up-and-coming young players. He saw very few of his suggestions for refreshing the squad materialise. For Bayern, it promises to be a challenging summer of comings and goings. The two senior centre-backs at the club, Jerome Boateng and <a href="https://www.thenationalnews.com/sport/football/david-alaba-announces-that-he-will-be-leaving-bayern-munich-at-end-of-season-1.1167439">David Alaba</a> will leave when their contracts expire in June. Alaba is most likely to join Real Madrid, while Boateng weighs-up his options having learned only recently – Flick was angry about the timing of the announcement – that Bayern would not be renewing his deal. Boateng and Alaba have been lining up in the Bayern defence together for a decade, have done so in two victorious European Cup finals and, if Boateng can overcome soreness in his knee, Flick will value their combined experience at the Parc des Princes. Bayern’s defensive alignment was utterly awry for the opening half-hour of the first leg, when Kylian Mbappe scored on the break and a Neymar pass put Marquinhos clear to score PSG’s second goal. Boateng then replaced Niklas Sule. The comeback Bayern launched from that point will be their touchstone tonight. Goals from Eric Choupo-Moting and Thomas Muller took a see-saw first leg back to 2-2, before Mbappe struck the third PSG away goal. On that wild night, Muller lost count of the chances Bayern racked up in the Munich snow. He thought it was 20 by the final whistle. Statisticians counted 31 Bayern shots. “When you have 31 shots, you still have to add some coolness with the finishing,” said Muller yesterday [Monday]. “But we’ll be happy if we get that many opportunities again. We just have to make better decisions when they come.” “It’s about keeping the balance,” added Muller of the task before Bayern. “We have to take some risks, and our basic philosophy is to attack. We will push for an early goal, but we don’t need to go out straight away trying to crowbar it in.”