Scans have revealed the 35-year-old midfielder damaged his L4 vetebrae during the second half the Fratton Park fixture following a collision with team-mate Clark Robertson, whose knee accidentally caught the Irishman in the back.
The former Millwall man will have a further cat scan tomorrow to reveal the true extent of the injury, with the layoff time not yet known.
Meanwhile, former Portsmouth, Ipswich and Wigan boss Paul Cook will not be making a return to management with National League side Chesterfield.
The Derbyshire Times claim talks between Cook and the Spireites took place place overnight, but an agreement could not be reached.
It comes after the club suspended manager James Rowe pending an investigation.
No further details have been revealed.
Elsewhere, Sheffield Wednesday’s next opponents Burton Albion have been fined £4,500 for failing to ensure their players conducted themselves in an orderly fashion in their draw with Wimbledon.
The charge relates to an incident in the third minute of the game, which took place on Saturday, 22 January.
New Brewers loan signing Christian Saydee could make his Burton debut when they take on Sheffield Wednesday at the Pirelli Stadium.
The 18-year-old forward has joined the club as a replacement for Daniel Jebbison, who was recalled by Sheffield United late in the transfer window.