The 32-year-old, involved in a loan swap deal with Bantams winger Omar Daley in February, has not featured for City since sustaining the injury in training nearly seven weeks ago.
Interim manager Peter Jackson said: "There's no point keeping him here with his current injury situation. From what I've been told, there seems little hope that he'll play before the end of the season.
"He's been suffering with his thigh problem for the past nine games or so and with him looking extremely unlikely for Saturday, it was best for all parties if he returned back to Rotherham.
"He's a great senior pro and he's been a great lad to have around the dressing room."
Ellison scored the winner on his Bradford debut in the 1-0 home win over Wycombe in February and went on to make a further six appearances.
Source: PA
Source: PA