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.
Bayer Leverkusen have bolstered their attacking options with the acquisition of South Korean forward Son Heung-min from Bundesliga rivals Hamburg.
The west German club are resigned to losing Andre Schurrle after reaching an agreement to sell the highly-rated 22-year-old to Barclays Premier League outfit Chelsea.
Leverkusen have acted swiftly in an effort to strengthen their squad and have signed Son, who has 15 international caps, to a five-year deal for an undisclosed fee.
"With Son Heung-min, we have a committed player who already was among the top performers, despite his youth at HSV," said Leverkusen chairman Wolfgang Holzhauser.
"Son fits in our requirements as a young, still very viable professional with international standards. We are happy that he decided on Bayer Leverkusen as the club with which he can realise his own lofty goals."
Son had been with Hamburg since 2008 and rose through their youth teams before going on to become an integral part of their first XI.
He scored 12 league goals to guide HSV to a top-half finish last season and will be looking to make his mark with Leverkusen.
"Heung-Min Son is exactly the type of player we were looking for our young team. He is fast, agile, technically strong and has a great shot," added Leverkusen sporting director Rudi Voller.