* Vilanova could be out for around six weeks
* Reports say Abidal returns to full training (Adds details)
By Elena Gyldenkerne
BARCELONA, Dec 19 (Reuters) - La Liga leaders Barcelona have been rocked by news coach Tito Vilanova is to have surgery on his saliva glands on Thursday, followed by chemotherapy and radiotherapy over the following six weeks.
"While monitoring his parotid gland a change has been detected in his condition that requires surgery," Barca said in a statement on Wednesday.
The 44-year-old stepped up from assistant to replace Pep Guardiola at the end of last season after having surgery on a tumour in his saliva glands in November last year.
No news was given on who might stand in on an interim basis with the side still to play Valladolid away in La Liga on Saturday, before the two-week mid-season break.
Local media speculation has centered on his assistant Jordi Roura or even the return of the out-of-work Guardiola.
"His time in hospital will be about three to four days," Barca said. "During the period after his operation, depending on how he recovers, an attempt will be made to balance his treatment with his working life."
A news conference with Barca president Sandro Rosell and sports director Andoni Zubizarreta will be held at 1845 GMT, Barca said.
The news about Vilanova's illness had been filtering out during the day, drawing messages of support from around the soccer and sporting world.
"Real Madrid wish to express their love and support to Barcelona coach Tito Vilanova, to whom we wish a prompt recovery," Barca's arch-rivals said in a statement.
"Real extend this support to the club and his family."
Former tennis world number one Rafa Nadal said: "All my strength and support for Tito Vilanova! We are all with you to overcome this next hurdle."
Vilanova was out of action for around three weeks after surgery last year, but returned to the dugout to help Guardiola for the last six months of the 2011-12 campaign.
The quietly-spoken Catalan, at his presentation in June, said the experience had left him with a different perspective on life.
"I had an important operation a few months ago," he said. "I spoke with the medics and my family about the job, but they all said I was perfectly okay and there was no problem.
"Having overcome this operation it gives me a different view on life. Perhaps, after what has happened, training Barca will be like child's play."
Vilanova has guided the team to the top of La Liga with 15 wins in 16 games, nine points clear of second-placed Atletico Madrid, and into the last 16 of the Champions League as group winners.
He is the second member of the Barcelona squad to be afflicted with such an illness, after France defender Eric Abidal had to undergo surgery to remove a tumour from his liver in March 2011.
The 33-year-old returned to the field of play later that year but also suffered a relapse and underwent a liver transplant in April, from which he has yet to return to action.
Reports said Abidal has been given the all-clear to return to full training and had joined up with the rest of the squad for the first time on Wednesday - in a session which Vilanova did not take. (Writing by Mark Elkington in Madrid, editing by Mark Meadows)