The former All Black captain gave Williams the jersey he wore in his final test for inspiration; Williams made the move back to New Zealand in 2010 and played for Canterbury and the Crusaders - and the All Blacks - before moving to the Chiefs.
Umaga has also been a big supporter of Williams' boxing, and there are suggestions the 29-year-old code-swapper will put on the gloves again soon.
Umaga was ringside at Williams' fight when he won the New Zealand heavyweight title against Clarence Williams in 2012, and posted on his website afterwards: "Sonny is a great friend of mine and a phenomenal athlete. He has so much motivation, drive and dedication that he deserves all the success he gets ..."
Coach Hansen, watching from Johannesburg, will probably feel heartened by Williams' willingness to play rugby, if not the boxing part of his plans.
The big second-five is looming as a key part in Hansen's midfield equation for next year's World Cup. To hear that he is so keen to get back into the 15-man game having barely paused for breath after the Roosters' elimination from the finals is likely to be music to his ears.
It is also a boost for the national provincial competition, a once-loved institution that is falling deeper into insignificance by the year.
Top players these days use it only as a rehabilitation device - witness Dan Carter's 40 minutes for Canterbury in their defeat to Southland on Sunday.
For Williams to want to play so soon after the rigours of the NRL is a credit to him, the work ethic Umaga has written about, and Umaga himself.