Last season's top scorer has been sidelined since October with a heel problem which eventually required surgery.
But Alexander has resumed training this week and manager Kenny Jackett says he will include the striker if he is fit enough.
Alexander's return means that, for a change, Millwall have a clean bill of health going into the derby clash.
Fellow frontman Neil Harris has shaken off his recent back problem and should start in attack alongside 18-goal leading scorer Steve Morison.
The Lions have won 11 of their last 13 matches to move onto the shoulder of second-placed Leeds.
Kick-off is at 7.45pm.