The boss will consider recalling Tommy Elphick from his loan stay at Hull, while Brentford centre-back Yoann Barbet is also understood to be a target when the window opens next month.
Villa have lost Manchester United loanee Axel Tuanzebe to a foot injury, with James Bree drafted in to play alongside James Chester in the heart of defence.
And their vulnerability at the back was ruthlessly exposed by a Leeds team who climbed back to the top of the Championship, overturning a two-goal deficit to win 3-2 thanks to Kemar Roofe’s stoppage time strike.
Smith admitted the return of Elphick was a “possibility” and added: “We always knew we were light in defensive areas. Unfortunately Axel has been injured.
“James Bree came in at centre-half but he has played 30 or 40 games at Championship level.
“We have enough defenders but we also know it is something in January we need to deal with.”
Tuanzebe, who had been in impressive form, is expected to miss a lengthy period and has returned to United for treatment.
Villa led 2-0 after just 17 minutes thanks to goals from Tammy Abraham and Conor Hourihane.
But Leeds fought back in the second half through Jack Clarke and Pontus Jansson, before Roofe won it at the death.
Villa, the Championship’s second top scorers, have now conceded 12 goals in their last four home games.
And Smith said: “We have conceded too many goals, especially here at Villa Park. We have drawn 5-5 against Forest when we should have won the game and drawn 2-2 against Stoke and lost the game today conceding another three goals.
“We have to make sure, as a collective, we are better at keeping clean sheets. That gives you a good platform to try and win games. With the firepower we have at the club, we will score goals. Defensively, we have to be better.”