Mullins tore ligaments in training back in April and was expected to be absent for a minimum of 6 or 7 months.
But the utility man is making rapid progress to be back fit sooner rather than later.
"I'm told I’m ahead of schedule which is good news but there's a lot of work still to do,” said the 25-year-old, who has just started jogging to strengthen his leg.
Meanwhile boss Andy Scott confirmed to Radio Sheffield that striker Liam Dickinson is not in his plans after taking a look at the front man during a short trial spell. Rumour had linked the free agent to the Millers after he left new club Plymouth after only 17 days.
A reminder the Millers send a youth team to Stocksbridge tonight, 7.45pm start.