Christian Day has backed James Haskell to achieve his aim of going to the World Cup by starring for Saints next season.
Haskell has signed a one-year deal at Franklin's Gardens as he looks to keep himself in contention for an England call-up for the big tournament in Japan next year.
The 33-year-old, who has won 77 Test caps to date and toured with the British & Irish Lions in 2017, will bring a vast amount of experience to the Saints back row following his departure from Wasps.
And Day, who has decided to call time on his rugby career as he starts life as a Rugby Players Association (RPA) player liaison officer from July, believes Haskell will be a big success at Saints.
"James is a personality that divides opinion, but I've never had a problem with him personally," Day said.
"He's an infectious guy, he's larger than life, but what you don't see is that he's one of the hardest working guys. He really does take his fitness and his body incredibly seriously.
"At the end of the day, he's a British Lion. He's an England international, he's played in Super Rugby, he's played all over the world.
"If you forget about all the smoke and mirrors around him and just look at the nuts and bolts of what he does, he's an incredibly reliable performer, he's very hard working and he's done everything there is to do in the game.
"The rumours are that he's come slightly cheaper than expected because he really wants to put himself in the window for the World Cup, he wants to play for England and that's a driving force for him.
"If he wants to get back in that England team, this is his opportunity, and I've no doubt, knowing the guy he is, that he'll do that."