Bruno Senna said he knew it was likely that Valtteri Bottas would replace him at the Williams team for the 2013 season.
The Brazilian joined the Grove squad for the 2012 season, but was replaced by Bottas in 15 of the 20 opening practice sessions during the year.
Williams confirmed on Wednesday that the Finnish driver will race alongside Pastor Maldonado in 2013.
Senna admitted the announcement did not come as a big surprise for him.
"Since the beginning of my programme with Williams I accepted that I had to share the car with Valtteri for 15 Fridays as a part of his preparation for a likely debut in 2013," Senna said.
The Brazilian scored 31 points this year and finished the championship in 16th position, one place behind team-mate Maldonado, winner of the Spanish Grand Prix.
Despite losing his seat, Senna believes he has made huge progress as a driver this season.
"It has been extremely satisfying to be the team's most regular point scorer and for me to demonstrate my pace in all 20 races," he said.
"I have learned a lot and improved considerably through the season. I have had a relatively short career so far and feel as though I have come a long way in just a few years.
"I had some notable achievements this year, such as the fastest lap in the Belgian GP, my favourite track, scoring points on a regular basis and making up many, many places through the races.
"Regularly racing in the top 10 has certainly been a step forward in my still relatively short career and has allowed me to further sharpen up my race craft.
"Working with a competitive and supportive engineering team like Williams has left me well prepared for the next step in my career."
Senna said he will announce his future plans soon.