The industrious loose forward hurt her knee during the Spirit's 38-10 loss to Wellington in Porirua last weekend.
She played through but coach Scott Manson said the flanker was getting a scan yesterday to check whether she had suffered ligament damage.
"It looks like she has done an MCL [medial collateral ligament]," Manson said.
"She is getting scans ... but it does not look very positive from the feedback from physios and the doctor."
Otago will host premier heavyweight Counties Manukau at Forsyth Barr Stadium tomorrow.
Zoe Whatarau will start at openside in Mason's absence. It is one of two changes to the loose forward unit.
Rebecca Todd and Tegan Hollows will switch spots. Hollows started at hooker last week but moves to No8.
Jess Kendall gets a start at lock at the expense of Kate Smith, who drops to the bench.
Prop Eilis Doyle has been ruled out with an ankle concern and
Aparima College year 13 pupil Amy Rule will start in her place.
Kilisitina Moata'ane is back running after picking up an ankle complaint during the pre-season. She may make her playing return as soon as next week.
Midfielder Islay Fowler is no closer to making a return from concussion, though.
She started feeling ill during the lead-up to the game against Hawke's Bay on September 8 and is still "not very well".
The Spirit was well-beaten by Wellington last week but Manson said his side defended well in the first half and would need to find that intensity tomorrow.
Otago team v Counties Manakau
Sheree Hume, Kiana Wereta, Georgina McCullough, Greer Muir, Trisha Hopcroft, Rosie Kelly, Rosie Buchanan-Brown, Tegan Hollows, Zoe Whatarau, Morgan Henderson, Julia Gorinkski, Jess Kendall, Amy Rule, Rebecca Todd, Isla Pringle. Reserves: Gemma Millar, Paige Church, Kate Smith, Rachel Scott, Sammy Wong, Amy Du Plesiss, Cheyanne Cunningham.
