Zakuani trained on Thursday but is struggling with a groin injury on which he is still having treatment. The 23-year-old complained of soreness after the session and will be assessed prior to kick-off against the Hornets.
Other than Zakuani, boss Mark Cooper has no other injury concerns.
Paul Coutts had been out with a shoulder complaint but returned for the 3-2 defeat at Coventry last weekend.
Cooper’s side is also free of suspensions, although influential forward George Boyd is one booking away from a one-match ban.
Peterborough are rooted to the foot of the Championship table, seven points from safety, but could leapfrog Plymouth if they claim at least a point this weekend.