Bayern, who lost to Borussia Dortmund in the German Cup final last week, are eyeing their fifth European crown when they take on Chelsea in Munich. Eight of Bayern's squad will then join the national team, preparing in Italy, on May 25.
"If they lose again in a final, after their league and German Cup losses, in their own stadium... then disappointment would be great which I think would mean I would need to give them (Bayern players) two or three more days to process that," said Loew.
"They would then need a bit longer to find their motivation but they would find it. Because the Euros are also a huge target. But a defeat would be, I think, a bit of a low blow for us," he told the German football association in an interview.
Bayern keeper Manuel Neuer, defenders Holger Badstuber, suspended for the final, Jerome Boateng and Philipp Lahm as well as Bastian Schweinsteiger, Thomas Mueller, Toni Kroos and Mario Gomez have been called up for the tournament co-hosted by Poland and Ukraine.
"To win the Champions League title would be important, it would be an historic achievement," Loew said.
"That would be super for our players because they would join us fully motivated and would find their rhythm immediately because of that motivation."
Three-times European champions Germany, looking for their first title since 1996, have started preparations in Sardinia this week with less than half of the players called up.
The players from doubles winners Borussia Dortmund are arriving later on Tuesday following their 5-2 German Cup win over Bayern on Saturday, while Real Madrid's Sami Khedira and Mesut Ozil will join them in a few days.
"The preparation start is a bit unsettled. A training session on tactics is a bit difficult at the moment," said Loew, who will be in charge for his third tournament after leading Germany to a runners-up spot at Euro 2008 and third place at the 2010 World Cup.
"It is not easy, so we will have to see how we can do it. We are flexible, we will make it but time is very tight."
Germany take on Portugal, Denmark and Netherlands in Group B of the tournament which kicks off on June 8.