Koeman was critical of Barkley after Monday's win Sunderland, when the England midfielder was taken off at half-time, but the response was noticeable.
"I try to be honest about players," he said.
"You can't always say what you think about players because you need to keep it inside but he played how I like to see Ross: offensively strong and defensively he worked very hard.
"Maybe in 90 minutes he lost less balls than in the first 10 minutes against Sunderland.
"We had some days after Monday. First of all you have to speak to the player and you need to support him and you expect a reaction.
"Players are not so stupid, they know when they play well and when they don't. He watched all the clips and we had a good talk and finally it is up to the player."