Somerville has signed a two-and-a-half-year deal with the Guinness Premiership side and will leave New Zealand at the end of the year following their tour to Britain and Ireland.
Hansen conceded the Canterbury and Crusaders stalwart would leave a big void in the All Blacks squad.
"Greg's done it all in New Zealand rugby," he said. "He's done it quietly but done it really well. We'll miss him as a player, a leader and a mate."
The 30-year-old front-rower, who can line-up on either side of the scrum, played at last year's World Cup after fighting back from a torn Achilles he suffered in 2006.
He became the most capped prop in New Zealand history when he faced England in Christchurch last Saturday to record his 59th appearance having made his debut against Tonga in 2000.
Somerville, nicknamed Yoda, made his Canterbury bow in the National Provincial Championship semi-final in 1998 and went on to become the fourth man to play a century of matches for the Christchurch-based franchise in the 2006 Super 14 semi-final victory against the Bulls.
Canterbury coach Rob Penney hailed Somerville's contribution to the New Zealand game.
"Every once in a while a player turns up with all the attributes to be a top professional; selfless, hard working, athletic, mentally and physically tough, skilful, passionate and possessing an unrelenting desire to win," he said.
"We've been fortunate to have such a player in Greg Somerville. His contribution to rugby at all levels has been immense, but above all, he's a good bloke."
Somerville joins a long list of players to depart following last year's World Cup.
Chris Jack, Aaron Mauger, Luke McAlister and Carl Hayman are already playing in the Guinness Premiership with Nick Evans due to link up with Harlequins for the new season.
Jerry Collins has signed for French club Toulon and Byron Kelleher is playing for Toulouse, who take on Clermont in this weekend's French championship final.