While Grzegorz Krychowiak pulled a goal back for Poland in the closing minutes, they were unable to find a second to rescue a point.
Lewandowski said errors and a failure to take risks cost his team, but he vowed they would be better in a crucial clash against Colombia on Sunday.
"Many things didn't go as planned. In fact, we let them score two goals. That's not how we take points," the Poland captain said, via L'Equipe.
"It becomes difficult to play when you have already given one or two goals and you have to chase the game.
"I think we didn't take enough risks. We played too conservatively and then Senegal waited. But above all, we didn't live up to our potential.
"We missed a lot of things. We made too many mistakes and of course when we make stupid mistakes, we become nervous.
"Against Colombia it won't be easy. We will play differently and better than today."
A 10-man Colombia were beaten 2-1 by Japan in their opening game, setting up a crucial clash between the sides in Kazan.