Stewart broke a bone in his right ring finger during the Robins' 2-2 draw with MK Dons on Saturday.
He said: "It sounds ridiculous that a broken finger can keep you out of action for a month but that's doctor's orders.
"I'm going to have a splint in place after the operation and wires coming out of the top of the finger so I can't play for fear of injuring an opponent.
"It's agony for me to miss out. I want to play for the club I love but for the next few weeks I will be restricted to watching from the sidelines."