Their £140million splurge showed a lack of foresight, planning, cohesion or strategy. While many lauded the Toffees' business there was a clear failure to replace Manchester United-bound Romelu Lukaku.
It's why the Everton faithful should be concerned about the prospect of Mazzarri being lined up for the manager's job if Koeman is sacked even if, on the face of it, the Italian coach has a good pedigree.
Mazzarri spent last season in charge at Watford but, despite keeping the Hornets up, was hugely unpopular at Vicarage Road.
Installed as Quique Sanchez Flores‘ replacement at Watford, the former Internazionale and Napoli head coach was seen as something of a coup for the Hertfordshire club who were heading into just their second consecutive season in the Premier League.
Things began reasonably well despite an unenviable fixture list with early clashes against Chelsea, Arsenal and Manchester United. The Hornets even won the last of that trio, beating José Mourinho's side 3-1 at home.
But that was as good as it got. Even after beating Leicester City 2-1 to climb to seventh in the Premier League in November there was a feeling Watford's position was false.
That victory followed a 6-1 humiliation at Liverpool. There was little shame in losing to a side with the Reds' attacking talent but the Hornets were listless. And it wouldn't be the last time.
A 2-0 defeat to Burnley a few weeks earlier had been the first worrying sign. The Clarets, back in the top flight after a season away, were nothing special; organised, hard-working and disciplined. Qualities Watford were woefully short of on the night.
Yet whenever they appeared on the verge of a truly cataclysmic run of form the Hornets always managed to pull something out of the fire. Between December 10 and January 29 they accrued just six points in the Premier League.
Then, out of the blue, they picked up a 2-1 win at Arsenal with new signing M'Baye Niang playing a starring role. The Milan loanee, who was at one stage over the summer an Everton transfer target, followed that up with a goal in the home win over Burnley.
Watford, boosted by the January additions of Niang, Mauro Zaraté and Tom Cleverley looked to have turned a corner. Once more it proved a false dawn. With safety now all but assured, the Hornets won just three more games and collected nine points from a possible 42.
Tools had well and truly been downed and the season ended with a limp, the Hornets lost six in a row and conceded 16. The culmination, a 5-0 capitulation against City, summed up everything that had been wrong with Mazzarri fielding a bizarre XI and two goalkeepers on the bench.
Yet when it was announced prior to the season's finale against City the coach would be moving on there was uproar among neutrals and pundits.
What did Watford expect? Survival was the most a club of their size could aspire to and Mazzarri had delivered that. But there was more to it than simply finishing above the dotted line.
Mazzarri was cold and impersonal. Despite spending time in Manchester earlier in his career in order to learn English he made little effort to demonstrate his linguistic skills.
That made it hard for supporters to warm to him in the same way they had Sánchez Flores which, given the disappointing collapse in the second half of the campaign, might have bought the Italian some patience if not time.
No grasp of English could have talked supporters around to the style of play Mazzarri's side played, though. The Hornets were direct, one-dimensional and, frankly, boring.
With just 40 goals bagged they were the fifth-lowest scorers in the league. Conversely, 68 against was the fourth-worst record, meaning only the relegated sides were more porous than Watford.
Furthermore, Mazzarri's tactical and selection decisions were baffling. He persisted with three at the back when it was apparent the Hornets were more solid playing a system with four defenders.
He always set his sides up to play on the back foot, cramming as many defensive-minded midfielders into the side as possible with Valon Behrami – his on-field lieutenant from their time together at Napoli – crowbarred in at every opportunity regardless of form, fitness and suitability.
While he might have been popular with the Swiss enforcer, Behrami was in the minority. Rumours of a rift with skipper and dressing room leader Troy Deeney persisted throughout the season and it was evident when Mazzarri was sacked that the players had not been completely enamoured with the former Coppa Italia winner.
Even the usually mild-mannered Heurelho Gomes hinted strongly that the coach's poor grasp of English had contributed to the team's disappointing displays.
“It is important to speak English if you are a manager,” Gomes told BBC Sport after the final-day humiliation at the hands of Manchester City. “They cannot pass on instructions to the players but he tried his best and we thank him for it.
“The club had to make a decision, and they did. The language didn't help him. We apologise for some of our mistakes, we can't just blame the manager.”
And there is an element of truth in what the Brazilian goalkeeper had to say. Mazzarri was not helped by the debilitating number of injuries the team suffered, Odion Ighalo's sudden goal drought or the club recruiting for a back three before it became apparent that wasn't working.
But the things the former Inter coach could control he struggled with and when the decision to part company became public there was a sigh of relief all round.
Everton supporters should exhale similarly if the Toffees dodge the Mazzarri bullet.