Mason, who also missed the first meet of the season against Kent in the National Trophy due to illness, will undergo hospital tests.
Against Stoke at Arlington on Sunday the IT First Eagles will use rider replacement while on Monday, away at Rye House, Dan Greenwood has been drafted in.
The Eagles are third in the National league table having won and lost one of their two meetings so far.
They beat King’s Lynn 64-24 at Arlington but lost on the road to early season league-leaders Cradley 48-41.
Rye House lost their only league meeting so far 49-39 at Birmingham.
Matt Bates has been drafted into the Eastbourne IT First Eagles team as a replacement for the injured Danny Warwick.
Bates captained the Devon Demons (riding at Plymouth) last year in the National League and comes recommended by Richard Andrews and his father, Gary.
Warwick is set for a lengthy spell on the sidelines after suffering a serious neck injury in last Sunday’s big win over King’s Lynn.
He has now left hospital in Brighton and returned to his Poole home.
Bates is 25 years old and began his speedway career on his 15th birthday with Weymouth.
He will come in as a second string, having ridden last year as a heatleader in the curtailed Demons’s season.
Last season Bates and Andrews were both in the Devon team.