Lewandowski started strongly but endured a fourth goalless match at Eur 2016, as Poland surrendered a lead to draw 1-1 in Saint-Etienne, reaching the quarter-finals after winning 5-4 on penalties.
But Nawalka, not for the first time in France this month, came out strongly in favour of his captain.
"Robert is doing phenomenal work for this team, he is incredibly important mentally and in football terms, he helps the team to be better, stronger, he motivates them," the head coach said.
"The most important thing is the victory for the whole team. Robert in these matches ... we know from history, there are some cases where players are the star of the team and when they couldn't score goals the result didn't go their way and they didn't score.
"But it's the opposite here, he's fantastic. We are very happy that he can fully, physically and mentally, play in every match, he is giving and creating chances for other players, opening up the space, creating good positions for them.
"He takes opponents' attention away from other players. It's great work. I'm confident and sure that he will get a break in the next match and then let our opponent be afraid."
Poland's players were pelted with objects after celebrating their shootout win in front of the Swiss fans at Stade Geoffroy-Guichard, prompting suggestions they intentionally provoked the supporters of their beaten opponents, having been jeered throughout the second half for perceived attempts at time wasting.
The Polish Football Association's media representative responded on behalf of the coach to questions about the squad's conduct, claiming it was a coincidence the team found themselves in that area of the field, having rushed to their goalkeeper Lukasz Fabianski after Grzegorz Krychowiak's decisive spot-kick.
Nawalka meanwhile gave short shrift to suggestions the coaching staff may have lacked belief in the team's capacity to reach the last eight after conceding to Xherdan Shaqiri's extraordinary overhead kick and struggling to cope with Switzerland in extra-time.
"Do I look like someone who could possibly be scared about the result or have any doubts? Of course not," he said.
"And that was the case on the bench. We were confident our game plan was going to work out. We always have great faith in our players. Our players have optimism ahead of every single match."