Mateta omitted from Palace squad to face Forest
AC Milan have emerged as frontrunners to sign the 28-year-old, who has also been linked with Forest, before the transfer window shuts on Monday.
"I can confirm JP will not be in the matchday squad," Palace manager Oliver Glasner told reporters on Friday.
"It's nothing right now to do with transfers, he's just not in the right place right now to play. To protect the team and protect him, that's why he will not travel to Nottingham.
"The second thing to confirm is there will be no new player in the matchday squad."
Palace have reportedly made a bid for Wolves striker Jorgen Strand Larsen, who could be a direct replacement for Mateta.
"The club is working hard, but there is no decision," said Glasner.
"To be really very clear, it can be that JP stays and is a Crystal Palace player on February 2, and it could be that JP has left the club and a new striker is in.
"For me, these are the two situations we are talking about and discussing right now."
Glasner confirmed earlier this month that he would be leaving the south London club at the end of the season.
Palace, who won the FA Cup last season, are winless in their past 11 matches across all competitions, leaving them eight points clear of the relegation zone.
They also suffered a humiliating FA Cup third round defeat at sixth tier Macclesfield.