Bayern Munich's director of sport Matthias Sammer criticised his team despite its 2-0 win over Werder Bremen.
Two late goals ensured Bayern made it six wins out of six and extended its lead at the top of the Bundesliga table to five points, but Sammer said the performance was still not up to scratch.
"What we did today up to the 1-0 was too little," he said. "You have got to be honest about it."
The match was Bayern's eighth of the season and fifth in the last fortnight and Sammer hopes the strain of such a busy schedule was the only reason for what he saw as a sub-standard performance.
"If it is normal for such a game to come given the rhythm, then we can take that, but if it is something mental, then we cannot accept that," he told Sky television.
"We have to evaluate the performance and not always the result."
"But in the end, it was enough for a win against a strong Bremen side," Heynckes added.
"However, I had to use some clear words at half-time."
Newly-promoted Hertha Berlin has strengthened its squad for next season's Bundesliga campaign by signing Japan midfielder Hajime Hosogai from Bayer Leverkusen for one million euros ($1.31 million).