Simpson was recently moved onto the club's long-term injury list after needing back surgery for the second year running.
"He's going to be a while, Dawson, unfortunately," Cats assistant coach Dale Amos said on Tuesday.
"He hasn't had much continuity at all in his last couple of years, so he will be pretty slow.
"It will take him a period of time to get back. It's unfortunate for him because he was looking to have a good run at it, and his pre-Christmas stuff had been quite good."
Geelong is expected to promote a player to fill Simpson's spot in the lead-up to its round one blockbuster against Hawthorn.
That means the club's rookies have plenty to play for during the NAB Cup.
"For all those guys that are on the rookie list, it's an opportunity for them to press their case to be on a senior list," Amos said.
"The most likely candidate that might play AFL footy … will emerge over the next couple of weeks.
"It might be determined by injuries a little bit as well, because we've got some different sizes and shapes with our rookies.
"So I guess it will be what we feel like we need at the time. That will determine who gets elevated.
"But I'd say that will happen closer to round one."
A 196cm ruckman/forward, who played seven AFL games last year, Walker would be a straight swap for Simpson.
But the fact that midfielders James Kelly and Steve Johnson are both unavailable for the Easter Monday clash with Hawthorn might sway the decision Sheringham's way.
Sheringham certainly showed he can compete at the highest level during his five AFL games in 2012, and the 24-year-old been an impressive performer on the track during Geelong's pre-season.
"Love the way he's training," three-time premiership player Paul Chapman recently told CatsTV.
"He's hard at it and uses the ball really well."
Geelong's other rookies are forward George Burbury, who is currently sidelined by a foot stress fracture, former steeplechaser Mark Blicavs, midfielder Cameron Eardley and ex-basketballer Ryan Bathie.
Blicavs lined up against Fremantle giant Aaron Sandilands in the second of Geelong's round one NAB Cup matches.
A player whose extraordinary endurance is his greatest asset, Blicavs competed strongly against Sandilands, winning his share of hit-outs and covering plenty of territory.
But he has only been with the Cats since his dream of competing at the London Olympics ended midway through last year.
As a result, the chances of him being elevated rather than the likes of Walker and Sheringham are slim.
"Obviously the second-year and third-year guys are the ones that are getting towards the stage of their careers where they need to push their case to be on an AFL list," Amos said.
"So Walker, Sheringham, Burbury – he's unfortunately injured – are the guys you'd expect to he pushing pretty hard to get an opportunity."
Josh Walker is a forward in AFL Fantasy. He averaged 43 points in 2012. Register your team at our AFL Fantasy hub.
Adam McNicol covers Geelong news for AFL.com.au. Follow him on Twitter at @AFL_AdamMcNicol
