The midfielder caught his studs in the turf in a challenge midway through the first half of Saturday's match with Derby County. After lengthy treatment he was eventually stretchered off with suspected ankle ligament damage.
Speaking on Monday, manager Gary Bowyer revealed that Marshall will need to be assessed further before Rovers can get an idea as to how long he'll be out of action.
Bowyer said: "He had a scan yesterday and he has got to have it re-scanned later on in the week.
"Like I said at the time, it probably looks like it will be a little bit longer than a couple of weeks that is for sure.
"The lad is obviously gutted with the injury because he was playing the best football of his life, but if anybody knows Marshy you can't keep him quiet for a while.
"He's buoyant straight away and that helps with his rehab already."
The first game Marshall misses is this Tuesday night's match against Barnsley.
Tickets will go off sale at Ewood at 4pm on Tuesday, after that they'll be on sale on the night at Oakwell up until the 7.45pm kick-off.
Prices are £23.00 Adults, £16.00 for Seniors (aged 65+), Under 18s and Students. £5 for Under 12s.
Fans should note that neither student priced tickets nor Under 12s tickets are available on the day of the game. Under 12s tickets MUST be purchased with an adult/senior citizen ticket.
Students MUST be aged 18 to 20 years and produce a valid student card with photo ID and proof of D.O.B on entry at the turnstiles.