Thomas Muller is showing signs of becoming "his old self" at Bayern Munich, according to coach Jupp Heynckes.

The Germany international struggled for form under Carlo Ancelotti and found himself in and out of the starting XI during the Italian's final few months in charge.

Muller, who appeared to criticise Ancelotti's methods when he claimed Bayern would have "good training again" following Heynckes' appointment, has enjoyed a new lease of life since the treble-winning boss returned to the club.

The 28-year-old bagged an assist in the 5-0 thrashing of Freiburg and scored in the 3-0 Champions League win over Celtic on Wednesday, and Heynckes believes his performance showed he is nearing his best once more.

"He's close to finding his old form," Heynckes told a news conference. "He started on Saturday and on Wednesday. He ran 13 kilometres against Celtic; of those 13, he ran one at a high intensity. This shows how important he was to us in that game and that he'll soon be his old self again.

"We have huge challenges and a lot of travelling in the next few weeks. I saw how Thomas sprinted and worked defensively; he was great. He was an example as a captain."

Bayern resume Bundesliga duties against struggling Hamburg, who have just two wins from their first eight league games and have not beaten the champions in the last 17 competitive meetings.

Heynckes, however, insists they are a team Bayern must not underestimate as they look to close the two-point gap to league leaders Borussia Dortmund and stay above third-place RB Leipzig.

"You have to respect the opponent as much as possible. Hamburg are much better than results have shown recently," he said.

"It's a three-way fight at the moment. Dortmund and Leipzig are, of course, very good teams. We can still work hard in some areas and then we'll be able to improve again.

"We need to play in Hamburg in the way we started. When I see how the team reacts, it makes coaching again a lot of fun."