Caulker was initially very doubtful for the game after limping out of the FA Cup third-round defeat to Sheffield Wednesday last weekend.
He suffered a hip injury following a collision with team-mate David James and returned to parent club Tottenham for treatment.
"It didn't look too good to begin with. I didn't think he would be available for this Saturday, but now it looks as though he might be," said boss Keith Millen. "I spoke to someone at Tottenham on Tuesday and he told me the scan was just a precautionary measure. I'm certainly not ruling Steven out of the Middlesbrough game."
More good news for Millen is the return to fitness of captain Louis Carey. The defender has been out for two months with a series of problems, but came through a reserve-team game this week and is available for selection.
Another defender, Christian Ribeiro, is a doubt after sustaining a hamstring injury against Sheffield Wednesday. Liam Fontaine is on standby to come into the side, while forward Danny Haynes could also be recalled.
Source: PA
Source: PA