While veteran forwards Franck Ribery and Arjen Robben departed at the end of their contracts, with the latter announcing his retirement soon after, James Rodriguez also left and returned to parent club Real Madrid.
With three important players out of the door, Lewandowski insists the club must replace the trio if they are to maintain their success and ward off the title threat of rivals Borussia Dortmund.
"I think we need three new players," he told Bild. "We lost three players in attack with Franck Ribery, Arjen Robben and James, and so far we have not had a [proven] new signing in that area."
Though Bayern are yet to bolster their midfield ranks, the club have been heavily linked with a move for Leroy Sane, who reports suggest would cost north of £130m (€141m/$158m).
The Premier League transfer window closes before the opening fixture on August 9, with Manchester City boss Pep Guardiola having revealed how he wants the winger to stay.
However, even if Bayern pull off a deal, Lewandowski is adamant it won't be enough and identified where else he feels the team needs strengthening.
"We need another attacking winger - even if Sane comes," he added, before suggesting "perhaps a No.6 or a central attacker."
However, the Poland international insists they need more and hasn't been afraid to express what he thinks to those in charge at the Allianz Arena.
With another Champions League campaign to contend with as they target European glory and domestic dominance, Lewandowski believes the team is short on numbers.
"If you want to play a whole season with 13 or 14 experienced pros, it's going to be hard," he said. "We have 50, 60 games over one full season, not three, four, five or six months.
"We have too few players for that, and [president Uli] Hoeness and [CEO Karl-Heinz] Rummenigge know my opinion."
Bayern are next in action on August 12 as they take on Energie Cottbus in the first round of the DFB Cup before opening their Bundesliga campaign at home to Hertha Berlin on August 16.