Before facing each in the Champions League, let's review the special relationship between Bayern's coach and Barça's star
1 - First permission. Pep Guardiola started his Barcelona career as a coach allowing Leo Messi to play the JJOO 2008 with Argentina. Barça had to play the qualifiers of the Champions League, but the coach decided Messi could go. Argentina won the Gold medal, Barça beat Wisla Krakow and Messi thanked his manager. Everybody happy and Guardiola started to win Messi's confidence.
2 - Messi, false 9. During Guardiola's first matches, Messi played as a right forward. Everything changed the night before the 2-6 at El Bernabeu. Guardiola was studying Madrid in his lab when, in the middle of the night, called Messi: "I've got something very important. Come here. Now", as revealed by Marti Perarnau in "Pep Confidential". Messi started to play as a false 9 the following day.
3 - From pizzas to fish. Until Guardiola's arrival, Messi had had several muscle injuries. When Pep arrives, physio Juanjo Brau becomes Messi's shawod and the Argentinian changes his diet. Less pizzas and more fish.
4 - Champions League. Back in 2006, Messi remained in bench in the Paris CL final despite having recovered from an injury. The Argentinian didn't like that decision from Rijkaard. 3 years later, in Rome, Messi won the Champions League scoring the decisive goal against Manchester United (2-0).
5 - Ibrahimovic's management. When season 2008-2009 ended, Guardiola decided to sell Samuel Eto'o due to "lack of feeling" and signed Zlatan Ibrahimovic. Despite a promising start, Messi didn't feel comfortable sharing the attack with the Swedish. In the last matches of the season, Bojan played as a right forward and Messi won the "centre forward battle". Ibra left the following summer.
6 - Explosion. "It was Guardiola who told me: with me you will score three or four goals per match", said Messi in 2011. His stats improved a lot. Before Guardiola, Messi had scored 42 goals in 110 matches. Since Guardiola, 363 in 365. Once he scored 5 against Bayer Leverkusen.
7 - Ballon d'Or. Finalists in 2010 where 3 Barcelona players (Messi, Xavi and Iniesta). Guardiola was in charge of delivering the prize. Iniesta seemed favourite due to the World Cup won with Spain, but Messi won in. Here's the historical picture of Guardiola giving the Ballon d'Or to Leo Messi.