ST KILDA ruckman Billy Longer will be sidelined for at least a month after suffering some concussion related symptoms.

The 23-year-old copped a knock to the head late in a VFL game for Sandringham on May 7, but passed all the concussion tests and played the following week.

The former Lion recently experienced some headaches from a bump at training, and was having trouble with his memory.

After consultation with specialists, the Saints have decided to take no risks and have ruled the big man out of competition and contact drills for four weeks.

"Billy reported some concussion related symptoms following a head knock in the VFL that have required further investigation," general manager of performance Jamie Cox told the club's website.

"Club doctors, in consultation with specialists, have recommended he spends the next four weeks on a modified program, out of playing and contact drills.

"With any concussion related incidents, player welfare is paramount. We will take a conservative approach with Billy and he will be monitored closely over the next month.

"He can continue conditioning work and modified non-contact skills, which will set him up for a strong second half of the season."

Longer has yet to feature at senior level this season with Tom Hickey holding down the No.1 ruck spot.