However, Penney is optimistic about his chances of returning this weekend, while Gregan should also be passed fit despite taking a knock on his ankle on Saturday at The Valley.
Blackman, on loan from Blackburn, is expected to be available having played for an hour in the reserve-team's 3-0 loss against Manchester United in midweek after four matches out with a foot injury.
Versatile full-back Andy Holdsworth is hoping to return to the first team within the next fortnight as he recovers from tendonitis in his left leg.
David Connolly is pushing to make his Southampton debut for Alan Pardew's side.
The Irish striker was an unused substitute at Southend last Friday after completing his free transfer to Saints, and proved his fitness by scoring for the reserves in midweek.
Young winger Kayne McLaggon also played for the second string, his first run-out since suffering a ruptured Achilles in February, and could be in Pardew's thoughts.
Veteran full-back Graeme Murty is therefore Saints' only injury absentee with the knee ligament problem he suffered against Birmingham in the Carling Cup in August.
Jacob Mellis and Jake Thomson should be back in contention after international duty, but Simon Gillett this week joined Doncaster on loan.
Sporting Life