The former Lyon centre-back declared earlier in July that his head is "already at Liverpool" as he fumed at the Saints' refusal of a £20 million bid one year after his arrival at St Mary’s Stadium.
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"A player shouldn't say something like that, it's not very proper," he told Fox Sports.
"We just have to wait how it develops. For now, Dejan can still play with us. I don't want to lose him but, if he leaves, he will only go for a top price.
"Ron would definitely be an option," Koeman confirmed when asked about the Villa defender, "although I think he won't be cheap right now. Well, let's just wait and see."
Koeman has already bolstered his forward options this summer with the signings of Dusan Tadic and Graziano Pelle.
Both Rickie Lambert and Adam Lallana have left the south coast club for Liverpool already this summer, while Luke Shaw joined Manchester United in a £30m deal.