"Matt, he just came off (last week) and said his thumb was sore - the doctor X-rayed it and he had a broken thumb.
"It's a bit of a shame because he was in great form."
In the only other change to the starting XV which thumped the Chiefs, Scott Barrett has earned a second-row berth ahead of Luke Romano.
Capped 31 times, Romano will make his 100th Super appearance from the pine.
"Luke's an incredibly giving bloke, a real ideas man," Robertson said.
"He's big, strong, a little bit old school in the way he plays but new school in the way he thinks, always challenging me around how we can do things better."
Elsewhere, the Crusaders backline remains unchanged, with David Havili at fullback, the fearsome combination of Jack Goodhue and Ryan Crotty in midfield and Richie Mo'unga and Bryn Hall lining up in the halves.
Wyatt Crockett - who announced on Tuesday that 2018 would be his final year at the franchise - starts up front alongside Codie Taylor and Michael Alaalatoa.
Robertson expected a tough challenge from the Cape Town-based Stormers, who can call upon Springoks including Siya Kolisi and Pieter-Steph du Toit.
Giant lock Eben Etzebeth, however, will sit out the fixture injured.
The Stormers lost to the Waratahs 34-27 last weekend.
"They've got big men, an internationally-experienced forward pack and they're quality, playing good footy," Robertson said.
"It's really important we keep our rhythm and momentum, a bit of consistency."
CRUSADERS: David Havili, Seta Tamanivalu, Jack Goodhue, Ryan Crotty, George Bridge, Richie Mo'unga, Bryn Hall, Jordan Taufua, Billy Harmon, Peter Samu, Sam Whitelock (c), Scott Barrett, Michael Alaalatoa, Codie Taylor, Wyatt Crockett.
Reserves: Andrew Makalio, Chris King, Oliver Jager, Luke Romano, Heiden Bedwell- Curtis, Mitchell Drummond, Mitchell Hunt, Manasa Mataele.
- NZN