Having already seen captain Steven Gerrard dismissed just 38 seconds after his introduction at half-time, Liverpool were already pushed for manpower and eventually went on to lose 2-1 in what could be a telling result in the top-four race.
Nine minutes after Daniel Sturridge pulled a goal back for the hosts at Anfield, Balotelli - already on a yellow card - and United defender Chris Smalling were involved in a tussle on the sideline.
Balotelli's momentum saw him end up alongside the advertising hoardings and right next to the front row seats.
And Leatherbarrow, sitting square in front of the action, said he felt it was his responsibility to calm the Italy international - to avoid seeing Liverpool go down to nine for the final 10 minutes.
"The way I saw it, Smalling sort of shoved Mario off the pitch and Mario grabbed him just to have something to grab on to," the supporter told the Liverpool Echo.
"I could tell from Mario's body language he was ready to react and I knew he'd already had a yellow card.
"I was saying, 'Calm down, leave it, leave it', and he was saying, 'OK, OK'. I'm only five foot, eight inches and he's a big lad so I was struggling to get my arms around him.
"Peter, the lad who sits next to me at the ground, helped me and he's a police officer who sometimes works at Anfield on match days.
"Afterwards all the lads were laughing and patting me on the back as well as having a go at Smalling."
