Liverpool have now lost four games on the spin after United won at Anfield thanks to a headed effort from Harry Maguire.
The Red Devils opened the scoring just 63 seconds into the match after a fantastic strike from Bryan Mbeumo, but the goal was overshadowed by controversy.
Liverpool were furious that Oliver hadn't blown his whistle, as Alexis Mac Allister was down after suffering a blow to the back of his head. The decision split fans who believed play should have been stopped.
However, Gallagher has since back referee Michael Oliver, who he believes made the right call.
“Is there a foul? No, because it's Van Dijk who comes in behind Mac Allister. Michael Oliver assesses it and says 'no foul' and play goes on very quickly.
“He's forced to break with and so is the assistant, so neither see Mac Allister behind them. Within five seconds, the ball is in the net. The first time they see Mac Allister down, the ball is in the net so they can go back. It's just an unfortunate incident.”
Had Oliver spotted the altercation, he could have paused the game and, as a result, prevented Mbeumo's goal. He did not however and The Reds head into their Champions League clash against Frankfurt desperate to end their losing streak in what has been a dire few weeks for manager Arne Slot.