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 32-year-old centre-back, who can number Chelsea, Stuttgart and Sporting Lisbon among his former clubs, has signed a one-year contract at De Kuip after leaving Brondby.
A statement published on Feyenoord's official website revealed Boulahrouz's deal in Rotterdam includes the option of a further year.
He left RKC Waalwijk for Hamburg in 2004 and made 35 appearances for Holland across eight years.