Stanislas went into a challenge with Luke Shaw with his studs up but despite awarding a foul to United, Scott failed to hand either a yellow or red card to the Cherries attacker.
United went on to run out 1-0 victors at Old Trafford, but Gallagher has suggested that Bournemouth were fortunate to end the match with 11 players.
Gallagher told Sky Sports News: "I wouldn't have been surprised if he'd been sent off. It's the height and the fact it's a stretched leg. What I would say is that it's a minimum yellow card and yet he didn't get any punishment at all whatsoever. It's gone from one extreme to the other.
"On that tackle, it had to be a yellow card if the referee felt the tackle was dangerous and only he would know the force he's gone through with.
"Only the referee can judge if that was serious foul play. Once you blow you have to judge whether it's careless, reckless or seriously endangering the safety of another player. I would say that's at least in the middle band."
Stanislas had been making his eighth league start for the south coast side this season.