O’Hara, on loan from Tottenham, has a stress fracture of two vertebrae at the base of his spine.
He had an injection earlier in the week to try to ease the pain, but may have to settle for a place on the bench.
Midfielder Marc Wilson is also doubtful with a hamstring problem, but Tal Ben-Haim has recovered from a groin injury.
Grant must decide whether to rely on the defensive qualities of Hayden Mullins at left-back or gamble on the attacking flair of Nadir Belhadj, who has had a hamstring complaint, to try to catch Chelsea on the break.
David James, meanwhile, has rued the fact Pompey’s league form could not measure up to their cup exploits.
“The FA Cup performances, bar that one at Coventry, have been exceptional,” said James. “That is probably the saddest thing, that we were not able to reproduce it in the Premier League.”
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