He told the Liverpool Echo: "We hope a few more faces will come in to boost our confidence and it will make us more alert because we will all be fighting for places.
"I hope Johnny is one of them. I know him really well - he's a good defender, a good professional and his record shows that as he's played for Ajax, Atletico and Holland.
"He'd be a great signing for us. He can play in a few positions - right-back, centre-back or just in front of the defence. He's a quality player."