Bayern head into the 2019-20 season as seven-time reigning Bundesliga holders but without Champions League success since 2013.
The German champions have already signed World Cup-winning French duo Lucas Hernandez (€80million) and Benjamin Pavard (€35m) following the departures of Franck Ribery, Arjen Robben, Mats Hummels, Rafinha and James Rodriguez.
But star striker Lewandowski insisted Bayern must splash the cash in their ongoing pursuit of domestic and European glory.
"We have to wait for more transfers to come in," Lewandowski told reporters in Los Angeles on the eve of Wednesday's International Champions Cup game against Arsenal. "It is difficult to win the Champions League without new signings.
"To play at that top level, you need certain qualities and a strong team to be competitive. We haven't really done a big transfer in the past two years. We need new signings."
"Everyone knows it's not optimal that we have so few players in the first team right now," the 30-year-old added. "We need more players for the competition in training. I hope something will happen in the next few days."
On recruits Hernandez and Pavard, Lewandowski said: "Players like Hernandez and Benjamin Pavard can only help us. They are versatile, both world champions and ready to play at the top level at Bayern and we can be successful with them."
Lewandowski's team-mate Joshua Kimmich expressed similar concerns.
"We have a lot of high-quality players, but is that enough in the end to be able to go into the season?" Kimmich told Sport1. "You can see it from how many youth players accompany us on the US tour. We currently have 17 first-team players.
"If you really want to tear it up in the Champions League, then maybe we still need some more [new signings]."
Lewandowski, who scored 40 goals in all competitions last season, is contracted until 2021 and Bayern chairman Karl-Heinz Rummenigge said the Poland international could extend his deal.
"We are in talks," Lewandowski replied when asked about negotiations. "I don't know how long it will take but we are on the right track."
"I feel really well, not only with the team but I also love the city. It doesn't only depend on me, there's other factors," he continued. "We still have to finish our negotiations.
"It's a little too early to say I want to end my career in 2023. If I get this extension, I would like to play longer of course. I still feel young. How long I want to play, I have to figure it out still. I'm not thinking about it now."