Clemence will definately miss the trip to Fratton Park having sustained a hamstring injury during the Reserves 1-1 draw with Wigan on Monday night.
Meanwhile Heskey, Jarosik and Melchiot all picked up knocks in the Premier League clash against Charlton last Saturday and remain doubtful for the game against Pompey.
Heskey lasted just ten minutes of the 1-0 defeat against the Addicks having sustained an ankle injury whilst Melchiot retired at half-time having sustained a knee injury. Jarosik was substituted with 14 minutes remaining having picked up a calf injury.