The guy who sits in another car is Russell- a man who has never driven an F1 race before and is affiliated to Williams for a shorter period of time than Kubica.
I think he deserves several more races before being judged just because even the best drivers have some really bad race weekends from time to time so we need a bigger sample. And there is some doubt about whether lack of new parts might affect the performance of his car. But his 8 years absence should not be an excuse when his team-mate is a rookie. And he knows the team better than Russell.
But Kubica is in fact a rookie too. All his knowledge about F1 from his first "stint" is useless because F1 has totally changed since 2010 when he was racing for the last time before his absence..
He actually had to start from the beginning and his knowledge about F1 from the past was not helpful.
Russel on the other hand had recent seasons of racing.