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.
Tottenham have confirmed Schalke midfielder Lewis Holtby will move to White Hart Lane immediately.
The two clubs have reached an agreement to bring forward the Germany Under-21 captain's transfer, which was due to go through in the summer.
A statement on the Spurs website read: "We are delighted to announce that we have reached agreement with Schalke for the immediate transfer of Lewis Holtby."
Tottenham were initially content to wait until the summer before welcoming the 22-year-old in to their squad, but after the German club made it clear they were willing to let Holtby go this month, Spurs swooped to secure his immediate transfer.
The London club had two offers rejected for the player, before Schalke accepted a bid of £1.25million over the weekend.
It is understood that Holtby had a medical at Spurs two weeks ago before finalising his Bosman deal with the club.
He has signed a four-and-a-half year contract. It is understood that Holtby wants to move to London from Germany at the weekend, ruling him out of Spurs' games against Norwich and West Brom, which take place on Wednesday and Sunday respectively.
Holtby was born in Erkelenz to an English father, Christopher, who was in the British Army, and a German mother.
He had spells at Sparta Gerderath and Borussia Monchengladbach before making his professional debut with second division side Alemannia Aachen six years ago.
Holtby joined Schalke two years later and his performances with the Gelsenkirchen club helped him earn him earn youth caps with Germany at Under-19, Under-20, and Under-21 level.
He could have played for England, but opted to represent his country of birth, making his full international debut against Azerbaijan in June 2011.
Holtby is a versatile midfielder whose favourite position is just behind a main striker. He can also operate deeper in midfield.
Speaking after agreeing to sign at the end of the season, Holtby said: "Tottenham are a top club and I'm fulfilling my dream.
"Negotiations with Schalke were initially advanced, but then I looked over it all carefully again. I have spoken to Andre Villas-Boas and I am confident in his plans."
Schalke director of sport Horst Heldt has wished Holtby the best of luck at Spurs.
"We have found the best solution for all involved," he said on his club's website.
"Lewis had hinted to us that he would not be against moving immediately, and Schalke have received a decent sum for a player whose contract was due to expire in five months.
"We wish Lewis all the best for the future."
Holtby's last appearance for Schalke, whom he joined in 2009 from Alemannia Aachen, came in Saturday's 0-0 draw at Augsburg.
"He always behaved correctly and gave it all he had got for Schalke until the very last day," added Heldt.
Lewis Holtby factfile:
1990: Born September 18 in Erkelenz, Germany, to an English father and a German mother.
2007: Makes professional debut with Alemannia Aachen in the 2. Bundesliga.
2009: Joins Schalke on a four-year contract.
2010: January - Moves to Bochum on loan until the end of the season.
May - Joins Mainz on loan for the 2010-11 season and helps the team finish fifth in the Bundesliga.
November - Makes Germany debut in a goalless draw against Sweden.
2011: July - Wins DFL Supercup with Schalke, scoring a penalty in a shootout win over Borussia Dortmund.
2012: May - Finishes third in the Bundesliga with Schalke.
December - Announces he will leave Schalke at the end of the season.
2013: January 4 - Tottenham confirm Holtby will move to White Hart Lane in July.
January 28 - Tottenham announce they have reached an agreement with Schalke for the immediate transfer of Holtby.