Arsenal have up to €20million (£17.8m) to spend in the January transfer window, according to reports. Arsenal are linked with a host of players as Mikel Arteta looks to replenish his squad. However, Arteta is keen on offloading players first.
Left-back Sead Kolasinac and defender William Saiba have left the club on loan, with goalkeeper Matt Macey moving to Hibernian on a permanent transfer.
Mesut Ozil looks set to leave at some point this month, with Turkish side Fenerbahce favourites to sign the former Germany international.
And Greek centre-back Sokratis Papastathopoulos is attracting interest from across Europe.
But Arsenal are likely to only receive nominal fees, if anything, for the likes of Ozil and Sokratis, and are just keen on shifting their wages off the books.
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Arsenal transfer news: Mikel Arteta doesn't have much to spend on new signings (Image: EXPRESS)
"Arsenal, their priority in this window in midfield has been a creative midfielder," Castles said on the Transfer Window podcast.
"Basically someone who is a cross between a No 8 and a No 10, so a box-to-box midfielder with creativity and pass creation skills.
"I'm told they're working on a very limited budget to do that of about €20m.
"So you want a player who would take a fundamental role in the team and change the midfield in an important way but you're trying to do it at a price that is going to be very difficult to achieve."
Arsenal are linked with a move for Brighton midfielder Yves Bissouma.
Castles says the Mali international does not fit Arsenal's attacking-midfielder criteria but is a long-term option.
He added: "But Bissouma's not that player, he doesn't fit that style, but we've talked about how Arsenal have a very heavy squad, a lot of unhappy players, a lot of players in there that Mikel Arteta is not convinced about and would like to shift out.
"In that regard, you can see Bissouma as an option. Premier League ready, he's had his adaptation period, a very difficult first year where I'm told he wasn't particularly focused on football, the way he was living wasn't the best possible in terms of being surrounded by friends rather than having close family there.
"That's changed and he's proved himself in the division."