The keeper has been in commanding form since taking over from Adam Bogdan – who also remains sidelined with a knee problem.
Lonergan, pictured, hurt himself making a save from Yann Kermorgant in Saturday’s 1-1 draw with Charlton Athletic and will be closely monitored over the next 48 hours to see if he is able to face Barnsley on Boxing Day.
Dougie Freedman was hopeful the 30-year-old stopper would make the trip to Oakwell.
“His elbow is bruised so it will need to be rested for a couple of days,” the Wanderers boss said. “We are hoping it will be fine for Thursday.”
Bogdan is still thought to be a fortnight away from full training following knee ligament damage.
If Lonergan is not passed fit, Freedman’s only other senior option is Arran Lee-Barrett, who signed a short-term deal as cover when Bogdan was initially ruled out, and a one-month extension yesterday.
The 29-year-old has played games for Wanderers’ development squad but has not been in competitive action since April 2012, when he turned out for Ipswich Town against Doncaster.