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.”