Celtic have agreed a fee with Dinamo Zagreb for defender Jozo Simunovic but they face a race against time to sign him before the transfer window closes, according to Sky sources.
The 21-year-old has been permitted to discuss personal terms and undergo a medical but he will not arrive in Scotland until Tuesday.
Simunovic's fee remains undisclosed, although reports in Scotland claim it is in excess of £4m and possibly as much as £5.5m.
Virgil van Dijk has been linked with a move away from Celtic and Southampton have been mooted as potential suitors for him for several weeks.
But no deal has been done there as yet and it is understood Simunovic is not coming in specifically as a replacement for the Dutchman.
Van Dijk, who was named in Holland's provisional squad for this week's internationals then omitted from the final pool, refused to commit his future to Celtic after their Champions League exit.
Celtic have insisted they don't need to sell the 24-year-old, recruited from Groningen in 2013, and are now set to spend instead.
Simunovic, a Croatia Under-21 international, has made 52 appearances for Zagreb in the last two seasons, scoring three goals.