The Scottish midfielder was taken to hospital for treatment after being forced out of Friday night's 1-0 first-leg win at Bramall Lane due to a rib injury and the club are waiting for scan results.
Teenager Callum McFadzean replaced Robson and could start after scoring his first senior goal shortly after the interval.
Striker Jonathan Forte is hoping to overcome the hamstring strain which kept him out of the first leg and could also come into contention for a start while defender Tony McMahon continues to struggle with an Achilles injury.
Manager Chris Morgan has no other major injury or suspension concerns as the Blades look to defend their slender lead and reach the League One play-off final for the second successive season.
- Sports & Recreation
- Bramall Lane
- Jonathan Forte