The German defender says the fringe players could now pay the price for Arsenal’s 4-2 FA Cup defeat at Championship club Nottingham Forest.
Banned Gunners boss Arsene Wenger was forced to watch from the stands as his side crashed out at the third round for the first time in his 21-year reign.
And Mertesacker, 33, says some wannabes have now shot themselves in the foot following their embarrassing exit.
He said: “It’s difficult to take because for those players who played, that was probably a last chance to show up and show the manager that they’re ready to play more games.
“It’s not the case anymore.
Arsenal (Odds: 7/2 to win, 8/11 to lose, 11/4 to draw)
“That was probably a last chance to show up”
Arsenal star Per Mertesacker
“The manager made a lot of changes, gave a lot of trust to players who haven’t played a lot recently, and I don’t think anybody justified their selection. That sums it up I think.”
Wenger, who has won the FA Cup a record seven times, couldn’t hide his hurt as his much-changed side were dumped out.
Mertesacker pulled no punches in his assessment, adding: “We were not precise, especially in the final third. We couldn’t create a lot of chances.
“In the second half there was a little bit more spirit from ourselves. But in the duels you could see that they out-battled us and that little momentum never really changed.”
Wenger had one eye on tomorrow night’s Carabao Cup semi-final first-leg tie at Chelsea.
But Mertesacker added: “We wanted the FA Cup. You couldn’t see that we really wanted to defend it.
“The third round is always the toughest. You face a Championship side away and you need to be ready. But for the majority of the game, we were not.”