Flex options help!

I’m in a must win situation for the playoffs and Kamara already pooped the bed for me this week.

I have these options and one flex spot. So far in my lineup I have
Beckham / Boyd / Chark / Deebo Samuel. Boyd is in the flex spot. I can switch one of the guys for Chris Carson on the bench. My gut says Beckham is gonna bust, but Deebo has a tough matchup.
This is PPR btw.

Any advice will help!