Bayern finished two points ahead of Dortmund in 2018-19 to retain their top-flight crown and have added six new players during the transfer window, which closes on Monday.
Coutinho arrived last week on a season-long loan from Barcelona and made his debut as a substitute in Saturday's 3-0 win over Schalke at the Veltins Arena, where Ivan Perisic also made his first appearance for Bayern.
Lewandowski, who scored a hat-trick to take his tally to five goals in two league outings this term, has been particularly impressed by the addition of Coutinho.
"His last pass is insane, he can be the key in important games, especially in the Champions League," he told Sport Bild.
"We become more unpredictable through him, that makes us more dangerous.
"He can play as the number ten, outside left, if necessary also on the right. We can play with different systems. We have more options now."
Bayern drew 2-2 in their opening game with Hertha Berlin and sit two points behind Dortmund, who have brought in the likes of Nico Schulz, Thorgan Hazard, Julian Brandt and ex-Bayern defender Mats Hummels.
Lewandowski previously expressed his dismay at his side's lack of transfer activity, but he is content with the recent signings and is confident the reigning champions boast more quality than BVB.
"Dortmund has done a lot, they have a big squad," he said. "But it's not about quantity, it's about quality. And that's for sure - we have the players with the higher quality."