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 Coca-Cola Championship table, seven points from safety, but could leapfrog Plymouth if they claim at least a point this weekend.
Watford will check on loan duo Henri Lansbury and Craig Cathcart ahead of the trip.
Arsenal midfielder Lansbury has missed the last two games with a hip injury and Manchester United centre-half Cathcart sat out last weekend's defeat by Derby with a knee problem.
Jon Harley will keep his place in the starting line-up if Lansbury is ruled out while club captain Jay DeMerit should start again at the back.
The USA defender started his first game since August against the Rams having recovered from eye surgery.
Teenage midfielder Ross Jenkins is still sidelined after undergoing a back operation.
Boss Malky Mackay and his side are playing under the cloud of possibly going into administration, which will see them deducted 10 points and plunged towards the relegation zone.
Sporting Life