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While the Crusaders looked into bringing Franks back into the squad, Robertson said they didn't have the space to facilitate it.
"If we had space and it had worked out, we'd have loved to have Owie back. He's been great to us for a long period of time, but it's a professional sport and the timing didn't quite work," he said.
"We went through the process. They have to obviously come home first. But with our contracting, he still had a year on his contract with Northampton so we weren't expecting it at all and we had a full roster.
"It's a professional game, and that's where the opportunity was. We appreciate that he's made that decision and it's best for him and his family."
Franks played 150 games for the Crusaders from 2009 to 2019, winning three Super Rugby titles with the club. He also earned 108 test caps for the All Blacks during the same 10-year span, and was part of the 2011 and 2015 World Cup winning campaigns.
In 2022, he will link up with the young group of props at the Hurricanes, featuring All Black Tyrel Lomax (25), and rising stars Alex Fidow (23) and Tevita Mafileo (23).
The Hurricanes are yet to announce the signing and it is unclear what his role will be, however Robertson said Franks' experience and knowledge would be a huge asset to the Wellington franchise.
"Owie is the ultimate professional. He understands how to get the best out of his body, knows his way around a competition – he's played plenty of them, and more importantly he's a world-class scrummager and he can hand down all that craft that he knows. It'll be great for the Hurricanes forward pack."