BBC reporter Phil McNulty has claimed that Everton "knew from March" that striker Romelu Lukaku would leave the club.
The Belgian forward joined Manchester United for £75m in the summer, and has made an impressive start to the season.
Everton, meanwhile, have been unable to replace his goals, and have found the net just twice in their opening five Premier League games.
And - finally for now - Everton knew from March Lukaku was almost certain to leave, Huge collective managerial failure to not replace him.— Phil McNulty (@philmcnulty) September 18, 2017
"Everton knew from March Lukaku was almost certain to leave," McNulty wrote on Twitter. "Huge collective managerial failure to not replace him."
He added that Jose Mourinho's assessment that Everton should be aiming for a top four finish was not an unrealistic one.
"Mischievous maybe but not sure Mourinho said too much wrong about Everton/Koeman having to have top four aspirations," he wrote. "He's just spent £130m.
"Point is given the rebuilding Koeman has undertaken & money spent it should at least be crossing his mind Everton must aim for the top four."
Everton have disappointed so far this season, despite their heavy investment, but Koeman has insisted that it is largely down to the difficulty of the fixtures.
"When you look at the fixtures at the start of the season, we knew we wouldn't win them all," the Dutchman said, quoted by BBC Sport.
"We have played Manchester City, United and Chelsea away, plus Tottenham at home. Four title contenders, with the Europa League in between.
"Now we have four games at home in different competitions and we need to win. If you don't win it is better to stop."