Celtic boss Ronny Deila says keeping his best players will take priority over acquisitions in the transfer window.
Chelsea striker Fernando Torres is understood to be set for a return to Atletico Madrid, according to Sky sources.
Manchester City youngster Karim Rekik has joined PSV on loan.
The teenage defender, who made his Premier League debut in December, will spend the whole of next season in Eindhoven in his native Holland.
Rekik - an Under-19 international - spent three months at Blackburn in the spring and has previously been on loan at Portsmouth.
He had been due to travel to South Africa as part of the squad for City's two-match pre-season tour.