Alonso announced earlier in the 2018 season that the Abu Dhabi Grand Prix would be his last in F1; with the Spanish driver making the switch to IndyCar.
The Polish driver is making an astonishing return to the sport after suffering severe injuries, especially to his hand, after crashing a rally car in 2011.
Williams will be determined to build a better car for 2019; after a disaster last season - with the team only managing to score eight points in total.
Kubica was asked for his reaction to comments recently from Lewis Hamilton and Alonso, who reacted positively to Kubica's return to F1.
"It's always good to have respect and I have respect for them," Kubica told RaceFans. "We know each other for a very long time, we have been racing in the past.
"It's a pity Fernando is leaving, we have a very good relationship on-track and off-track. But that's life. It will be an honour for me to race in Formula 1 again and I will try to do my best."