Everton drew 1-1 with Manchester City at the Etihad Stadium in their most recent fixture.
Everton lined up with Gerard Deulofeu, Yannick Bolasie and Romelu Lukaku in their starting line-up at the Etihad Stadium, as Koeman wanted to give City something to think about in defence.
The Dutch boss correctly predicted that City would line up with three central defenders, but Everton's failure to win the ball back very often meant that their attackers were starved of service.
Koeman, therefore, had to alter his tactic soon after the game started to ensure that they weren’t short of numbers while trying to defend.
The Dutch boss's in-game tinkering of his system appeared to pay off, as Everton picked up a 1-1 draw against Pep Guardiola’s side.
And Koeman explained: “I said to the rest of the staff that we need to be prepared for three defenders from Pep [Guardiola].
"We took a risk to start with three forward players. That is why we changed after 20 minutes to deal with that.”
Everton scored their goal against City through Lukaku, but they were unable to hold on to their lead.
Despite the best efforts of Maarten Stekelenburg, Nolito headed home an equaliser for the hosts to ensure that the points were shared.
Everton are now in sixth place in the Premier League table, after making an encouraging start to the new campaign.