Liverpool manager Rafael Benitez has made his first signing of the January transfer window after Slovakia defender Martin Skrtel completed his move from Russian club Zenit St Petersburg.The 23-year-old has agreed a four-and-a-half-year deal and will provide essential cover at centre-back with Daniel Agger still working his way back from a broken foot and the 34-year-old Sami Hyypia coming to the end of his Anfield career. Skrtel, who has 15 international caps, is believed to have cost the Reds a fee in the region of £6.5million - a club record for a defender. 'I think he is a player maybe not many people know about but he played against Everton for Zenit St Petersburg and he is a centre-half who knows the English style,' Benitez told the club's website, liverpoolfc.tv. 'He is aggressive, quick, is good in the air and I think he's a very good player for the future and also for the present. 'He is very competitive, but his mentality for me is very good. Watching him in some games his mentality is similar to Carra (Jamie Carragher) so I think it's very positive for the team. 'He has quality and [it will be] a good problem for me if I need to decide who to play in the future. He has experience and enough quality for playing as soon as possible.'