Robertson suffered a thigh strain during the warm-up for last week's draw against Kilmarnock and has failed to recover, while defender David Barron is also missing due to an ankle injury.
Buddies manager Danny Lennon does have David van Zanten available again after his recent suspension.
Ross County have a couple of injury concerns ahead of the clash with St Mirren at Victoria Park.
Manager Derek Adams revealed he would assess the unnamed players in question on Saturday morning.
The Staggies are still looking for their first point of the new campaign, but can move abover their opponents with a victory.