Just over a year ago Rob Holding made a £4m deadline day switch from Arsenal to bolster Crystal Palace's defence.

But a little more than 12 months on, when Palace issued a joint photo, external of the men's and women's squads to mark a club anniversary, the 29-year-old was nowhere to be seen.

Why has the two-time FA Cup winner's career come to a standstill?

Holding, who made 162 appearances for Arsenal over seven years, is currently training with the Eagles' under-21 team following a disagreement with manager Oliver Glasner.

"Rob is training individually at the moment," Glasner said on 1 September, when asked about the centre-back.

"We will talk together – he knows the reason. But it is something that stays between Rob and me. Nothing public."

Sources with knowledge of the dispute have told BBC Sport that there was a disagreement about a post-match running session following Holding's last involvement in the first team. He was an unused substitute in a 2-0 home defeat by West Ham on 24 August.

Holding has only made one appearance for the club since he joined on a three-year deal, in the EFL Cup last season against Manchester United, and has not featured at all in the Premier League.

The U21s train at the club's academy, a separate location to the first team's base, and sources on both sides say that the situation for Holding is unlikely to change.

Sources close to the player say Holding is accepting of that and intends to keep training professionally and being the best he can in the circumstances.

Palace signed three centre-backs this summer, with Maxence Lacroix coming in from Wolfsburg in a deal worth £18m, Chadi Riad joining from Real Betis in a deal worth up to £14m and Trevoh Chalobah arriving on loan from Chelsea.

The Eagles also kept hold of £70m-rated England centre-back Marc Guehi and have Chris Richards, Jefferson Lerma, Joel Ward and Nathaniel Clyne as options in defence. That means that if Holding did patch things up with Glasner, he would still potentially only be the club's eighth or ninth-choice centre-back.

Holding has deleted all references to Crystal Palace from his Instagram page and has unfollowed the club on the platform. He is still listed as a first-team player on the Palace website.

He played the full 90 minutes for the under-21s in their 3-1 victory over Gillingham in the EFL Trophy two weeks ago.

Holding is reported to have been an excellent role model at the academy, with one source describing his impact around the U21 team as "brilliant".

But the road to first-team favour seems like a long way back for the former Arsenal player, even with Palace in the relegation zone and winless from their opening seven Premier League games.