Lewandowski, who has been linked with a move to Spanish and European champions Real Madrid after changing agents which only fuelled speculation, has scored 20 goals in as many starts for Bayern in the Bundesliga this season.
The 29-year-old has netted more than 100 goals since arriving from German rivals Borussia Dortmund in 2014, while helping deliver three Bundesliga titles.
And former Bayern captain Matthaus hailed the importance of Lewandowski at the Allianz Arena, likening the Poland international to five-time Ballon d'Or winner Messi.
"Lewy makes a difference, he cannot be replaced," Matthaus said via the Bundesliga's official website.
"Robert is an important figure in Munich like Messi is in Barcelona. However, when Messi gets injured, the image of the Catalan game changes completely. Bayern does not have this problem.
"Lewandowski is a key player, but the team is not as set up for him as the Real Madrid game under [Cristiano] Ronaldo or the tactics of PSG under Neymar."
"Lewandowski really is an exceptional player," Matthaus continued.
"Lewandowski does not have an alternate who guarantees the same quality. The Pole is currently the best nine in the world. Bayern will not find another player like that."
The Poland international has scored 29 goals in 34 matches in all competitions this season for Bayern, who sit 19 points clear of second-placed Borussia Dortmund atop the Bundesliga table.