Michael Walker's bid to extend his lead in the Victorian jockeys' premiership hinges on an evaluation of a back injury.

Walker was stood down at Mornington on Monday after aggravating the injury which also forced him to miss rides at Moonee Valley two days earlier.

He will be examined on Tuesday to determine whether he can ride at the Flemington midweek meeting.

"He was in quite a deal of pain," Racing Victoria steward Rob Montgomery said.

"He hurt his back on Saturday and he's been taken to hospital for assessment.

"We won't let him ride again until he's been assessed by our doctor Gary Zimmerman."

Walker leads the state jockeys' premiership with 127 winners for the season, two clear of Dean Yendall, and has 33 metropolitan winners, putting him fifth in the Melbourne championship.