Yamal missed around eight weeks through injury before recovering in time for the tournament, and Lewandowski felt the lack of match fitness made life difficult for the teenager on football's biggest stage.
Speaking to ESPN, the Poland captain said Yamal's strongest performances came earlier in his career.
"Honestly, no, we didn't see his best this tournament. The best performance, the best Lamine for me, wasn't last season but two seasons ago. That was the best version of Lamine Yamal."
Lewandowski said it was never going to be easy for the winger to perform at his highest level after such a long spell on the sidelines.
"He's still young. He came to the World Cup after injury and he didn't play for like eight weeks, so it's tough. If you don't play for eight weeks and then have to start playing in the World Cup, you don't have time for training, it's just from game to game."
Despite that, the veteran striker praised Yamal's contribution and the way Spain's players helped him settle into the tournament.
"But he showed really good quality. The Spain players and the team helped Lamine, and Lamine also helped the whole team. I think that this cooperation was perfect."
Spain went on to lift the World Cup trophy, and Lewandowski admitted he was delighted for his former Barcelona team-mates.
"I know a lot of players from Spain and I spent so much time in Barcelona with them, so I supported them a lot."
He explained that his close relationships with the Spanish squad made it easy to cheer them on throughout the competition.
"Of course, I know from Argentina personally only Leo, but I know that he won the last World Cup. That's why I supported the Spanish national team because if you play with them and spend a lot of time together, it's easier to support this team."
Lewandowski believes Spain's World Cup triumph could be the start of an exciting era, with a young squad capable of achieving even more in the years ahead.
"I was happy because I know they were ready to win the World Cup. They are very young and the future is in front of them."
Barcelona have confirmed the signing of Germany international Karim Adeyemi from Borussia Dortmund on a five-year contract, with the winger completing a move he described as a childhood dream.
Scoring goals on football's biggest stage is the ultimate test for any striker, and the FIFA World Cup Golden Boot recognises the player who finishes as the tournament's leading scorer.
Arsenal have confirmed that William Saliba will be sidelined for an "extended period" after suffering a back injury during France's World Cup campaign, although the defender will not require surgery.
England's agonizing wait for a major trophy continues following a heartbreaking 2-1 defeat to Argentina in the World Cup semi-final.
