Rangers lost 2-0 to Hearts.
The defeat saw Hearts leapfrog Rangers in the Scottish Premiership table, and inflict a third league defeat of the season on Mark Warburton's side.
Rangers manager Mark Warburton
Stewart criticised Warburton's post-game comments, stating his confusion at Rangers failing to stretch the play and lacking a game plan.
He said: "I'm flabbergasted. Mark Warburton comes out and says they [Hearts] played the way we expected them to play, yet they still didn't seem like they had a game plan."
Warburton had also criticised the referee after disallowing the goal from Joe Dodoo, while admitting the offside call against teammate Harry Forrester was the right decision, said the officials took too long to make up their minds.
Rangers' Harry Forrester (L) celebrates scoring against St Mirren with Andrew Halliday
He told the Glasgow Evening Times: "Harry was probably offside but the fact is the goal was given. The flag doesn’t go up for 14 seconds and then the linesman walks five or six paces down the line.
"So what happened to change their minds? It’s not us being sore losers or clutching at straws but that makes the game 1-1 and that would have been an important turning point.”
Stewart hit back at Warburton's complaint, dismissing it as irrelevant.
He told BT Sport: "The decision was correct, so the timing is immaterial."
Rangers cruised to the Scottish Championship title last season but have found life in the top flight tougher. Currently they sit in third place, and host a dangerous Aberdeen side at Ibrox this weekend.