The 72-year-old replaced the sacked Carlo Ancelotti last week after a string of bad results left Bayern second in the Bundesliga, five points behind Borussia Dortmund.
"There is no time for familiarisation, Freiburg want to snatch points away from us," said Bayern forward Thomas Mueller, who is expected to benefit from Heynckes' return after languishing on the bench under Ancelotti.
Mueller was part of the 2013 treble-winning team under Heynckes, which also included the Champions League title.
"We parted in 2013 with the best of memories and that is where we will try to pick up now," Mueller said. "We know the coaching team. It is excited and well tuned. This can only help us."
Bayern wasted two-goal leads to draw 2-2 in their last two Bundesliga matches and were also beaten 3-0 by Paris St Germain in the Champions League, a defeat that prompted the departure of Ancelotti.
Heynckes has promised to shake the team out of its current daze and have players work for each other more.
"I already feel a real spirit of optimism and there is a good atmosphere within the team," Heynckes said on Wednesday after two days of coaching the team.
"I am confident we will have a positive attitude going into our next matches."
Bayern have a busy five weeks ahead with league action, the German Cup and Champions League matches, which could go a long way in determining their season.
Dortmund, who have yet to lose this season and are on 19 points with six wins from their seven matches, host last season's runners-up RB Leipzig.
Dortmund have conceded just two league goals and netted 21 this term, scoring 13 in three home Bundesliga outings and they are unbeaten at home in 41 league games.
Naby Keita's return from suspension could not have been timed better for Leipzig, who will desperately need the Guinean to try to control the midfield against the aggressive Dortmund.
After a rocky start Leipzig look to be hitting their stride with back-to-back wins over Eintracht Frankfurt and Cologne.
Third-placed Hoffenheim, level with Bayern on 14 points, host Augsburg.
(Reporting by Karolos Grohmann; Editing by Peter Rutherford)
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