The Toffees were beaten 2-0 by the Championship club in the third round of the EFL Cup after the manager handed starts to a number of fringe players.
The likes of Tom Cleverley, Aaron Lennon and Enner Valencia were among those given the chance to shine in the league cup but did not overly impress Koeman who is set to restore his first-choice starting XI for Saturday's Premier League clash with AFC Bournemouth.
"There's too much focus on these players. What I tried to do was put out the best XI that I thought was the best for the weekend," he said.
"They need to give competition to the rest of the players. Normally we have in every position two players.
"Some had a very good game and some had not the productivity. I was not happy about the result, of course, but I was comfortable with what they showed.
"It was not good enough because we did not score the goal but in general I wasn't disappointed about the work-rate of the team.
"Sometimes you miss chances, that's part of football. In cup football you don't get a second chance. That's a pity but we need to accept it and go on because we have had a good start to the season."