Quoting: cody_tugak
Letang, Manson or Ristolainen might be good temporary replacements, I know Ristolainen is probably a bad choice too but he does have size and I believe does play better than Zaitsev
I don't think any of those guys are real options. Kris Letang would be a great 2nd pair vet to bring Sanderson into the league with, but he probably wouldn't come here, and you probably don't want to be left holding the bag on a big retirement contract. Manson is a middling player who is likely gonna get a reputation contract that he won't live up to. Ristolainen is every bit as Terrible as Zaitsev, he's just bigger and puts up points. I wouldn't take him at league min.
If Ottawa is gonna spend on a replacement for Zaitsev, the bar needs to be "guy who is comfortably a solid top 4 guy", you can't settle for "better than Zaitsev" and you can't bet on older players aging like wine, because in Ottawa those bets rarely pay off. That said, I'm not sure anyone on the upcoming UFA market fits that role for Ottawa. They might need to look at trade options.
It's certainly not something you can reliably bank on, but I think Ottawa needs to invest more into Europe, and try to find those 23-25 year olds who might be able to come right over.
Also Ben Roger is super intriguing as a mobile 6'4 RD, but he's probably several years away from sniffing the NHL. He's probably another four years (two more junior, at least two AHL) from that point. He's certainly not a guy to pin your hopes on at this stage.