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.
The 21-year-old moved to Stamford Bridge two years ago after struggling to force his way in to first-team reckoning at Barcelona.
Romeu, who has represented Spain at every youth level, has played 33 times for the Blues but missed the second half of last season with a serious knee injury.
He will now link up with Valencia ahead of the forthcoming season as Chelsea continue to send their squad players out on loan.
Romeu joins the likes of Marko Marin, Thibaut Courtois and Patrick van Aanholt in penning temporary moves away from west London but Romelu Lukaku, who impressed whilst on loan at West Brom last year, is still likely to remain a member of Jose Mourinho's squad at Chelsea.