Recent loan signing James O'Connor was stretchered off with a ruptured thigh muscle during the 3-2 Sky Bet League One defeat at Swindon at the weekend.
That has prompted O'Driscoll to try and bring in a right-sided player ahead of Saturday's home clash against Colchester United.
"We need another defender in," said O'Driscoll.
"James Wilson and Liam Fontaine cannot be recalled in the first month, so it's back to the loan market. We'll definitely have to have a look at what's out there and see if we can do something.
"It will be a tall order to get someone in before Tuesday night's game (against Southampton), but Colchester on Saturday will be a realistic possibility.
"The right-side of defence has always been an area we are a bit short in and Jimmy coming in helped plug a few gaps.
"He can play left-sided and right-sided centre-back and wing-back on both sides, so we are going to miss him."