FORMER Collingwood defender Jonathon Marsh is in line for a shock round one comeback after officially signing with St Kilda on Thursday.

As flagged by, Marsh underwent a medical at Moorabbin on Wednesday before committing to terms as a readymade replacement for Jake Carlisle.

Forced to open a list spot to add the rebounding defender, the Saints have placed injured draftee Jack Bytel on the long-term injury list for the season.

Marsh, 23, will provide instant defensive depth after injuries to Carlisle (back) and Oscar Clavarino (ankle) and doubt around Dylan Roberton's health.

Before Marsh's arrival, other Saints defensive options included Irishman Darragh Joyce, forward-turned-defender Josh Battle and Logan Austin, who played VFL on the weekend.

Key defender Nathan Brown is also serving a suspension.

After two strong seasons for East Fremantle in the WAFL, Marsh switched to Richmond VFL for this season and has impressed in senior intraclub matches at Punt Road.

Should he be named for round one against Gold Coast, it would come 938 days since his last AFL match for Collingwood in 2016.

The West Australian played 15 matches for the Pies before retiring prematurely at the end of 2016 for personal reasons.

Returning to Melbourne this season, the 194cm tall recently signaled a strong intent to return to the top level.

Marsh is the latest player to join club ranks via the new pre-season supplemental selection period (SSP) rules before Friday's deadline.

Hawthorn added beanpole ruckman Ned Reeves on Thursday after Sydney plucked forward/ruck Hayden McLean out of the SANFL on Tuesday.

It follows Carlton's recent move to add Williamstown dynamo Michael Gibbons and Dandenong Stingray Matthew Cottrell last month. 

Meanwhile, Gold Coast is expected to add former NAB AFL Academy member Sam Fletcher before Friday's period closure.

Other clubs with remaining list spots – Carlton, GWS, Melbourne and Port Adelaide – will hold over positions for the mid-season draft in round 11.