The Brazilian, who had been ever-present in Formula 1 since 1993, lost his seat at the end of last season - which was also Williams' worst ever performance in their history.
Barrichello has since moved to IndyCar, but, the 40-year-old has not given up hope of returning to the F1 grid.
"I think there is still a way back," Barrichello told Auto Moto und Sport. "My heart bleeds that I cannot be there.
"I think it's a shame for Williams because I think would have had a really big year. Not just for me, also for the team. Williams would have benefitted from me.
"I would have been a good team-mate as a coach for Senna, Bottas or Maldonado. Next to me they would have gone much better than they can now.
"Look at Maldonado. He had few accidents last year than this season. Pastor is super-fast and with me at his side he would've been a lot more controlled.
"Okay so they have more points to his tally than last year but with this car we would have been in the top third."