Masi was relieved of his duties after the controversial final round in Abu Dhabi, where Red Bull's Max Verstappen prised the world drivers' championship title away from Lewis Hamilton with a final-lap pass of the Briton for the race win.
Mercedes lodged an ultimately unsuccessful protest, then withdrew its intent to appeal following the announcement that the FIA would investigate the circumstances of that race and how the Safety Car period was managed.
Niels Wittich and Eduardo Freitas are now the race directors, a role which they will serve on an alternating basis, while Masi is set to take up a new position within the FIA.
That the latter was removed from the position has left a bitter taste in the mouth of Horner.
“Was it right to fire him [Masi] based on pressure that was placed on him from a rival team? That for me was wrong,” he told BBC Sport.
“That's tantamount to bullying. It's passively aggressive.”
Horner's comment is taken as a reference to the threat which seemed to exist that Hamilton would quit F1 due to what transpired at Yas Marina.
Neither attended the FIA prize giving ceremony despite F1's sporting regulations compelling the drivers' championship runner-up to do so.
Hamilton also entered into a protracted absence from public engagements, save for collecting a knighthood, and social media.
Horner's comments to the BBC have been published around the same time as the release of a Sky Sports (Britain) documentary about the intense 2021 title battle in which his Mercedes counterpart Toto Wolff claims that Red Bull sporting director Jonathan Wheatley had developed a “bromance” with Masi.
The Australian had used similar words to Wheatley's radio calls in justifying to Wolff how he managed the Safety Car period which set up the one-lap dash to the chequered flag, a week after his supposed ‘offer' about a Verstappen position drop created controversy when that exchange was broadcast on television during a red flag period.
“You've got to report the facts,” he stated.
“Who was the first to call Michael? It wasn't me.
“I'm only responding to the pressure being applied on him that I can hear in my ear from a rival team. It's my job as the principal of the team that I represent to defend it.
“I think it was probably less than the pressure that our rivals were pushing on to not have a Safety Car. Or to back-track a lap. Or not to have a Virtual Safety Car, or for the Virtual Safety Car to go into a full Safety Sar.”
Also notable in the saga is that the FIA not only replaced Masi, but also committed to a reform of how race direction works.
On that theme, Horner remarked, “Yes, Michael did make mistakes and it was frustrating, but you have to look at the role that he was in and the tools that he had at his disposal.
“You can't just place the blame on Michael. It's unfair to do that.”