Kerrin McEvoy could be out of the saddle for up to a month with a knee injury.

Scans have shown Darley's No.1 rider has a grade two medial ligament tear after hurting his knee in a race fall in the Karaka Million in New Zealand.

McEvoy was unhappy with his fitness after riding in trackwork and barrier trials in Sydney and Melbourne on Tuesday.

He immediately stood himself down from riding this week, missing Saturday's all-important CF Orr Stakes meeting at Caulfield.

McEvoy's main Caulfield ride would have been Earthquake in a Blue Diamond Stakes Prelude.

But results of Thursday's MRI scan now put McEvoy in serious doubt for the Group One Blue Diamond at Caulfield on February 22.

"MRI scan has shown a grade 2 tear to medial ligament. Physio and strengthening it up next few weeks," McEvoy revealed on Twitter.

"Physio says normally 2-4 weeks for this type of injury."