Lampard is not underrated. He was a phenomenal passer of the ball, with a lot of Chelsea chances coming directly from his creativity. He was also a workhorse in a similar fashion to Beckham, so he wasn't just allowing others to work, while he took pot shots.
This will sound wild, but Bergkamp.
Why?
He was a great player. However I think due to how aesthetically pleasing his game was, people really overrated his impact.
There were large stretches particularly from the 99-2000 season onward when Bergkamp was benched or not even considered a starter.
Yet people act like he was far more to Arsenal ( 01-06) than he actually was. They tend to stretch his prime when talking about him and moments like the goals against Newcastle and Leicester are used as testament, despite the fact that he never really had that level of consistency in his game.
If he played today, he would be far more scritinized than he has been.
In addition, I genuinely feel that if he played with the team that Ozil had to play with, people would not really see much difference between them.
He also faded in big games, he also wasn't impactful in key moments, he also wasn't a strong impact player, yet he always gets put into that upper echelon of talent. People regularly compare him with Cantona ( with lots of fans saying he's better), when in reality he benefitted from the fact that Arsenal were not as big as they are today and the coverage was not as much as it is today.
Yorke's United career was scrutinized for a bad 00/01 season, Cole was scrutinized for an average start to his United career. In today's game, Neymar is scrutinized for not winning the UCL with PSG, but more importantly not being available in key moments that could have made that possible.
Bergkamp chose not to partcipate in a lot of the UCL away games that led to Arsenal not doing anything impactful in Europe. Yet he has not been scrutinized for it, in fact it's even added to his mystique.
Great player, but far more flawed than people like to remember