Mateta, 28, is Palace’s top scorer this season with eight Premier League goals but told the club he wanted to leave earlier in January.
“Again, there is so much noise, so much speculation, so many things going on,” added Glasner.
“We need every single player at their best and JP [Mateta] isn’t at his best right now with all the noise that happens.
“It can be football noise, but if someone has private issues where they can’t focus on football, then it makes no sense to play.
“He can’t help the team and perform at his top level. We decided it is better not to play him.”
Palace sit 15th in the Premier League and have already sold captain Marc Guehi to Manchester City during the winter window.
They are yet to find a replacement for the England international but Glasner, who said earlier this month that he will be leaving at the end of the season, is confident the club are working to bolster the squad before Monday’s deadline.
“The club is working almost 24 hours a day right now,” Glasner said.
“Marc [Guehi] left two weeks ago and nobody is in. You can imagine it is not so easy – every team wants to play a successful season, so it is not so easy that you can get the players at the end of January.
“I see the club working hard to get players in to replace Marc and, of course, we have to prepare if something happens with JP [Mateta].”
Glasner said the club is preparing for different scenarios and Wolves striker Jorgen Strand Larsen remains a target.
“To be clear, it can be that Mateta stays, and he is a Palace player on 2 February, and it can be that Mateta has left the club and a new striker is in,” said Glasner.
“These are the two situations we are talking about and discussing right now.”
Palace were ready to offer £50m for Norway striker Strand Larsen, but that deal is now in the balance after the Eagles communicated their intention to walk away.
Wolves boss Rob Edwards confirmed on Friday that 25-year-old Larsen is ready to play against Bournemouth on Saturday.