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.
Celtic have secured the signing of Holland Under-21 defender Virgil van Dijk.
The centre-back has signed a four-year contract after Celtic agreed a deal with FC Groningen reported to be worth up to £2.5million.
The 21-year-old arrived in Glasgow for talks on Thursday and leaves Groningen two years before his contract was due to expire.
A statement from Celtic read: "Celtic are delighted to announce that defender Virgil van Dijk has joined the club from FC Groningen.
"The Dutch Under-21 international has signed a four-year deal with the Scottish Champions, and he becomes the latest player to join Neil Lennon's squad during the close season following the arrival of striker Amido Balde from Portuguese side, Vitoria Guimaraes.
"Virgil van Dijk will be officially unveiled at Celtic Park this afternoon at 2.45pm."
Van Dijk said: "It feels very good and I'm very excited to be here.
"Celtic is the biggest club in Scotland and they also play in the Champions League. They're a fantastic club and when Celtic came in for me I was so excited. Everything here is good so I can't wait to get started.
"I hope to play many games for my country in the future but first of all I want to make things happen here."
Asked what he will bring to Neil Lennon's double-winning side, van Dijk said: "I'm a strong defender and I'm good in the air, but I also like to play football on the ground, and I hope that we'll do even better than we did last season."