Rees-Zammit left Monday's practice with a back issue, Matt McMullen of the Chiefs' official site reports.
The former rugby star is likely fighting for a depth role on the Chiefs' 53-man roster, so this injury could hurt his chances of making it past cutdown day. Expect Mecole Hardman and Nikko Remigio to see even more reps returning kicks with Rees-Zammit sidelined.