Virgil van Dijk has been ruled out for up to three months
He went down under a challenge from Jamie Vardy and he now looks set for a long spell out.
He will miss the EFL Cup final against Manchester United on February 26.
Saints failed to snap up a new defender during the transfer window despite also selling club captain Jose Fonte.
— Southampton FC (@SouthamptonFC) February 2, 2017
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The club's only recognised centre backs are now Maya Yoshida, youngster Jack Stephens and Florin Gardos who has missed the best part of two years with injury.
"We studied a lot of solutions for a new defender, but we could not find a good opportunity," Puel said.
"Ryan Bertrand is a possibility for centre-back, but we have to look at all of the options in the squad."
Virgil van Dijk is set to miss three months of action
Virgil van Dijk as injured against Leicester and faces three months out
Southampton lost 2-1 to Swansea on Tuesday and sit 12th in the Premier League table.
They have already played three times without Van Dijk, only winning the EFL Cup semi-final against Liverpool.
They were also beaten 5-0 by Arsenal in the FA Cup.