Mertesacker himself scored Arsenal's first goal to level it 1-1 in the first half, but the German struggled badly to cope with the pace of Forest's teenage striker Ben Brereton throughout the match. And the defender, who gave the ball away in the lead-up to Forest's fourth goal, criticised the entire team after the match.

"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 today," Mertesacker told the Arsenal website in a frank assessment. "That sums it up I think."

Wenger rested most of his regular Premier League starters for this game, with Mertesacker playing alongside Rob Holding, Mathieu Debuchy and Ainsley Maitland-Niles in a back four that couldn't contain Forest's attack.

Mertesacker is retiring at the end of the season, meaning this was the last appearance for him in a competition he has helped Arsenal win three times over the past four years -- including a star performance in last season's final against Chelsea.

It also remains to be seen whether players like Debuchy and Theo Walcott will remain at Arsenal after this month's transfer window -- with the lack of more FA Cup matches possibly increasing the chances of them moving elsewhere.

"It's difficult to take because for those players who played today, 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," Mertesacker said.

Arsenal are already out of the Premier League title race but still have two trophies to play for, in the Carabao Cup and Europa League.

"Yes but we wanted the FA Cup," Mertesacker said. "This feels like we had to defend it, but you couldn't see that we really wanted to defend it. This 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."