Kershaw has adjusted his approach to pitching as his velocity has declined. He's not what he was 2011-2017 but still really good. I'm far less worried about him this postseason because the Dodgers are much less reliant on him. In reality he's their 3rd best starter. The only way the Dodgers would beat Boston last year was with Kershaw carrying them and it didn't happen. That doesn't have to be the case in 2019.
Jansen concerns me far more. he is what he is and with 3-5 mph off his fastball it opens him up to problems. I would rest him as much as possible and simultaneously build up his strength as much as possible these next 3 weeks.