The striker was taken off at halftime with the score locked at 2-2 at Manchester City, before the hosts ranout 5-2 winners on Saturday.
"I told him (Mateta) at half-time that it's a tactical decision," explained Glasner. "And I didn't really talk about it anymore because he knew it, he accepted it.
"We have quite a close relationship and of course I told him that he would start again tomorrow (against Newcastle).
"So there was nothing really to talk about. I know you take off your number one goalscorer at half-time and it's maybe big headlines for some, but for us it was not really an issue."
"I was disappointed with myself"
Of his tactical decisions at City, Glasner also said: "The season is quite long, with many games. We all have to work now until the end of the season. I think I'll start to recover on May 27, but for now it's just about being always switched on and getting the best (from the team).
"But of course after the City game I was very disappointed with myself. I was very disappointed about conceding five goals.
"It's the same for me. But I also have to find the turnaround. When you speak in front of the team it makes no sense for me to be disappointed, because you want to give energy to the players, they need our support, and I mentioned it after the game.
"It's not just about saying 'you did this wrong.' It's now just showing them what we can do better, what we already did better and finding the best solution for us as a team.
"You are the one who gives the energy to the team and this is what you have to find as the manager, where you can get this energy back as quick as possible."