The 25-year-old, who has only recently returned from long-term hip and shoulder issues, suffered the fresh injury during his side's 24-19 defeat at Gloucester last Friday.
It was hoped that Mills would not be facing too long on the sidelines, but Worcester coach Gary Gold confirmed that it will be the end of October before he can consider him for a first-team return.
"Ryan has got a reasonably significant tear in his quad, but hopefully it won't take too long to heel," he told Worcester News.
"He is pain free, but it doesn't make sense in risking him playing with that injury.
"I think it will be four weeks (before Mills returns)."
It is the latest blow for the side after fly-half Tom Heathcote also currently ruled out of action because of concussion.