During Kubica's time with the team, Williams have finished last in the constructors' championship in the last two years, going from 7 points in 2018 to 1 point in 2019, courtesy of Kubica during July's German Grand Prix.
"I think as a group, with me involved, we haven't done enough in the last 24 months," Kubica commented. "I am here exactly more than 24 months and when you sit down and George is commenting the same comments as I'm highlighting.
"When you look at the first five laps of the Williams car in Abu Dhabi, then it shows we have not done enough in the last 24 months, and I am saying it as a group because I have been and still for a while a part of this group.
"I think, from a work point of view, that this is the biggest disappointment. From my point of view, we should've reacted a lot quicker instead of thinking that there are no issues."
Speaking after the Abu Dhabi Grand Prix two weeks ago, Kubica said it is impossible to see if he is as good as he was prior to his accident in 2011, as he hasn't had a car that he can prove his talent is still present.
"I think it's very hard to answer this question," Kubica explained. "That's because you have to be in the position to see if you are as good as you were. I think this often wasn't the case this year.
"When you are racing in the front it's different when you are racing behind or in the back. From operation or walk point of view, I feel even more disappointed from my results point of view but that's my own personal view."
Bahrain International Circuit - Winter testing