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.
Hamburg's Hakan Calhanoglu is set to turn down an offer from Bayern Munich to join Bayer Leverkusen this summer, according to his agent.
Although he only recently penned a contract extension at Hamburg until 2018, he is said to be keen to leave the club after they only just avoided relegation this season.
Bayern and Leverkusen are interested in the Turkish playmaker, who favours the latter option, if Hamburg agree to sell.
"The next step for Hakan has got to be to a club where he can play - they cannot be on too high a level for him," Calhanoglu's agent Bektas Demirtas told Sport1 television.
"It's all about the overall situation. After a very difficult season, the lad has managed to increase his market value by an incredible amount.
"Now we're looking at next season and asking how his development is going to go.
"We want a quick resolution."
Calhanoglu is keen on playing in Europe next season, something Hamburg cannot offer, while Leverkusen qualified for the play-off round of the Champions League after finishing fourth.