Only 13 months earlier, the Mexican had been anointed as the successor of Lewis Hamilton, the team's most successful and recognisable driver in recent years and was charged with taking McLaren into a bright new era.
After only 17 grand prix weekends, however, the team had seen enough.
They decided that his target of winning the world championship, as he explained to Sky Sports' Pete Gill on his very first day at McLaren's headquarters, was little more than a pipe dream.
Perez had enjoyed a decent enough season with the car he had been provided—some handy races, some poor races—but was not considered to be a McLaren man.
A good grand prix driver? A podium contender? Of course.
A race winner? Possibly.
But multiple race winner? A world champion? A multiple world champion?
Not in the slightest.
McLaren's decision to reshuffle their driver line-up, then, revealed as much about their hopes for Magnussen as their disappointment with Perez.
As reported by BBC Sport's Andrew Benson at the time, Magnussen, who had just sealed the Formula Renault 3.5 title, was thought by the team to be a future champion of the world.
McLaren's suspicions were given increased credibility after the season-opening Australian Grand Prix, where the Danish driver inherited second place after an assured drive from fourth on the grid.
The ease of his performance led to obvious comparisons with Hamilton, the last graduate of McLaren's driver development program, whose similarly impressive debut at the Albert Park circuit in 2007 was a precursor for what was to come.
The struggles endured by both driver and team since Australia, however, have cast Magnussen's campaign in a more modest light.
His failure to consistently get the better of Jenson Button, who is by no means renowned for his blistering one-lap speed, in qualifying—the pair are tied six-six in the head-to-head records on Saturdays—reflects poorly on the rookie in a sport in which younger, fresher drivers tend to be the fastest.
That Button, the 2009 world champion, currently leads his teammate in the drivers' standings is perhaps no real surprise given his vast experience, although the margin between the pair is substantial, with Button is six points shy of having twice as many as Magnussen.
The claim of Eric Boullier, McLaren's racing director, in May that Magnussen, according to a McLaren-Mercedes phone-in (h/t Formula1.com), was suffering from "rookie syndrome" suggested that the Dane is the right driver at the wrong time for a team who are stuck in a rut.
For a group of people as meticulous and diligent as McLaren—and especially Ron Dennis, the unforgiving Group CEO—Magnussen's performance in last weekend's Belgian Grand Prix would not have gone unnoticed.
Although the 21-year-old enjoyed one of his finer weekends since the Australian Grand Prix at Spa, one particular moment of his race will have been most concerning for the team.
On the 42nd of 44 laps, a group of cars, headed by Magnussen and containing world champions of the calibre of Fernando Alonso, Button and Sebastian Vettel, charged their way up the Kemmel straight towards Les Combes.
As Alonso looked down the inside of Magnussen, the Dane swerved across the circuit, forcing the Ferrari driver onto the grass—a move that led to the McLaren driver receiving a post-race penalty of 20 seconds and two points on his super license.
Exactly 12 months ago, another McLaren driver was punished for a similarly overly-aggressive manoeuvre against another car on the very same piece of tarmac.
On that occasion, it was Perez at fault for running Romain Grosjean, the Lotus driver, out of road and defending his position far too forcefully, with the Frenchman left with no option but to use the asphalt run-off area on the exit of Les Combes.
Within a month of Perez being handed a drive-through penalty for that unnecessary incident, the first real signs of his vulnerability at McLaren began to appear.
The team's then-team principal, Martin Whitmarsh, was quoted by Sky Sports' Pete Gill as admitting McLaren's interest in Fernando Alonso for the first time.
And on the same weekend, at the Singapore Grand Prix, Perez told reporters in the official FIA press conference that his stay with McLaren for 2014 was "pretty much done"—a remark that was, of course, soon proven wide off the mark.
That Magnussen, like Button, is yet to be confirmed as a McLaren driver for 2015 suggests that the team now have reservations over just how far he can go in F1 and whether the gamble to replace Perez has truly paid off.
The wisdom of dumping a young driver after only one season would be highly questionable to say the least, but McLaren do have a record of doing so—and are, in fact, far more likely to do so when, in their current transition period, even the team seem unsure of the direction they want to take.
If Magnussen was rated as a world champion in waiting, his main assignment for 2014—like Perez a year ago, you suspect—was almost certainly to establish himself as the team leader and effectively finish the career of Button.
And although Button is still widely considered the most vulnerable to any incomings at McLaren for 2015, the prospect of the team chewing up and spitting out a young upstart for the second season in succession is an ever-increasing possibility.