The winger suffered a thigh strain towards the end of Saturday's 1-1 FA Cup third-round draw at Hull.
Boss Russell Slade has not yet ruled Cox out of the tie but Jimmy Smith, who replaced him at the KC Stadium, is on standby to start.
Lee Cook was feeling a calf injury at the weekend and could be rested. Neither Cook nor fellow QPR old boy Martin Rowlands have appeared in the competition so far.
Scott Cuthbert, Ryan Brunt, Gary Sawyer and Anthony Griffith played in the last round against Northampton and are pushing for recalls having been on the bench at Hull.