Lewandowski continues to be linked with a move to Real Madrid at the end of the season, despite Bayern chairman Karl-Heinz Rummenigge insisting the striker is not for sale.
"I know the Spanish media, there is a lot of speculation," he told a news conference. "I assume that Lewandowski will break my Bundesliga goalscoring record in the next two or three years."
Heynckes's comments follow on from those earlier in the week of Rummenigge, who insisted there was no reason for Bayern to sell Lewandowski.
"I like to assume that Robert is one hundred percent going to play at Bayern Munich next season," he told Kicker.
"What should motivate us to take a different direction?
"I always like to remember the year 2008, when we had an amazing offer from Chelsea for Franck Ribery.
"From that day on, the whole world of football knew, nobody can buy a Bayern Munich player against the will of Bayern Munich."
Meanwhile, Bayern resume domestic duties against Borussia Dortmund on Saturday, knowing they can secure their sixth title in a row if results go their way.
Jerome Boateng has been passed fit for the match after picking up a knock in Germany's friendly defeat to Brazil this week, but Arturo Vidal and Juan Bernat will miss out.
"Vidal has really bad bruising in his thigh. He won't be available," said Heynckes. "Bernat has slight inflammation in his knee, so also won't play. Otherwise, everyone is fit.
"[Boateng] took part in training as normal. All is good with him."
There was also more positive news concerning Manuel Neuer, who stepped up fitness training during the international break as part of his recovery from a foot injury.
"Neuer will increase his running load and continue to work in the gym," Heynckes added. "The injury has healed."