The 28-year-old banged in 23 goals in 104 games over four years with second-tier side Kiel, who lost a two-legged play-off against Cologne last May to miss out on promotion to Germany's top flight.
Champion Bayern Munich has a lower limit of 20,000 fans at its home games because of concerns of the spread of the delta variant, the dpa news agency reported.
Bayern president Herbert Hainer confirmed the forecasted loss after so many games behind closed doors, but insisted "our ambitions have not diminished".
Leicester City said Patson Daka will join on a five-year contract starting July 1, subject to league and international clearances.
Borussia Dortmund starts its title challenge against Frankfurt on either August 14 or 15 and Leipzig visits Mainz the same weekend.
Lewandowski scored in each of his last 10 league appearances, and clocked 41 goals in just 29 games altogether.
Union said the Berlin senate approved a pilot project Monday to have limited attendance for the game despite ongoing restrictions amid the pandemic.
Lewandowski converted an early penalty in Bayern’s 2-2 draw at Freiburg and Poland forward matched Gerd Müller’s record from 1972. Bayern secured the title last weekend.
As part of his penalty celebrations, the Poland striker lifted his match shirt to reveal a picture of Gerd Mueller over the text "4ever Gerd".
Schalke is already relegated but its last three games of the season are against teams with much still to play for.
Leipzig's defeat on Saturday means it can no longer overhaul Bayern, which holds a seven-point lead before hosting Borussia Monchengladbach in the late game.
Leipzig cut the gap on Bayern to seven points with three rounds remaining and can mathematically still win the title if Bayern follows its surprise 2-1 loss at Mainz with a collapse.
Schalke has condemned the fans who police said pelted players with eggs hours after the team was relegated from the Bundesliga.
Cologne appointed Friedhelm Funkel as coach for the last six games of the Bundesliga season on Monday.
In the interview published Monday, Petry criticized Hungary goalkeeper Péter Gulácsi, who plays for Hertha's league rival Leipzig, for supporting gay marriage and defending LGBT rights.