The Irishman scythed down the winger after coming on as a second-half substitute for Neil Lennon’s side, and the challenge sparked a midfield melee, earning cautions for Callum McGregor and Alfredo Morelos as well as Duffy for the tackle.
And the former Premier League official thinks the challenge was almost worthy of a red card in itself.
He said: “I think that’s as high a tariff yellow card as you can get - it’s bordering on red.
"Not so much for the tackle and the first leg it’s the fact he follows through. But the referee has seen it, an he’s just sent off [Nir] Bitton, and he needs to be convinced it has really has pushed over that boundary but as it’s right on it he’s given Shane Duffy the benefit of the doubt.”
The aftermath of the tackle also saw Alfredo Morelos earn a caution for his reaction and the referee was asked if Morelos should have been sent off having already been caught on camera catching Jeremie Frimpong late with a raking tackle down the full-back’s leg.
Gallagher told Sky Sports’ Football Show: “I think that’s a yellow card. When you’re on a yellow card and you make a tackle like that and don’t get sent off that’s when you can say the referee hasn’t followed through but he’s not on a yellow card when he makes his second so it’s a different challenge.
“When you see it, that’s a worse challenge than [Matt] Doherty made and that was a yellow card and he was ultimately sent off, but that’s a yellow card in itself.”
Gallagher agreed Madden got the decision to send Nir Bitton off correct, saying “For me it’s a red card.”
He explained: “Once Morelos gets past he’ll go straight towards goal an the law says ‘has he a goalscoring opportunity’?
"I would suggest he would get a shot away before [Kris] Ajer gets across.”
He added: “It’s a challenge Bitton doesn’t need to make.”