Lewandowski's contract expires at the end of 2022-2023 and there have been suggestions Bayern could look to move the Poland star on this year to free up room for Borussia Dortmund's Erling Haaland.
That is despite the 33-year-old having just won The Best FIFA Men's Player Award for the second year running, his 34 goals in all competitions this season at least 10 more than any other player in Europe's top five leagues.
Bayern CEO Oliver Kahn suggested on Wednesday that the Bundesliga champion has no plans to let Lewandowski leave.
"He is an absolute phenomenon," Kahn said. "Not just for the goals he constantly scores but for the level at which he has been playing for years.
"Of course, we will try to keep Robert with us. He's a kind of insurance for us, with his way of playing football. We will do everything we can to have Robert with us for as long as possible."
Sule, however, appears destined to leave the Allianz Arena after rejecting Bayern's offer of a new contract.
The centre-back, 26, has missed just two games in the Bundesliga this season and is reportedly of interest to Chelsea and Newcastle United, who could offer greater salaries.
"Negotiations with Niklas have dragged on for a very long time," Kahn said.
"We made him an offer, but he didn't accept it, as he told us. I think [club president] Herbert Hainer wanted to leave the back door a little ajar.
"Now we know what the situation is. This has happened over the past few days. I think you have to take into account that there are a lot of conversations going on. At some point, a player makes his decision.
"I think it was a very good offer. We have economic limits within which we must move. He's one of the best central defenders out there right now. I think we showed him that appreciation.
"Ultimately, it's the player's decision."
When asked if Bayern would look to sign a replacement, Kahn added: "We always look at all the options. We have a lot of options when it comes to the centre-back position.
"We have Lucas Hernandez in central defence, an excellent centre-back in [Dayot] Upamecano, young talent in Tanguy Nianzou. [Benjamin] Pavard can also play there."