Williams' change of heart and his late inclusion into the defending champion Kiwis squad at the expense of Melbourne Storm youngster Tohu Harris has sparked criticism in New Zealand that the player had become bigger than the team.
Tietjens was not among the critics but conceded the saga had been hard to digest.
"I really struggled with what happened," Tietjens told Reuters in an interview on Thursday.
"I feel in some ways for Sonny, but once he's made the decision not to make himself available, well, it should have been done and dusted.
"But the change has been made and it must be so, so tough on that (Melbourne) player."
Williams has shown disdain for long-term commitments since sensationally walking away from the Canterbury Bulldogs in 2008 to go play rugby union in France, while in the second year of a five-year deal with the Sydney-based National Rugby League (NRL) team.
The 28-year-old has signed a series of one-year deals, which have seen him return to New Zealand to play for two local Super Rugby clubs and clinch a World Cup with the All Blacks on home soil in 2011, while dabbling in professional boxing on the side.
Williams returned to the NRL this year to help the Sydney Roosters win the title on Sunday, but is widely tipped to return home to New Zealand for a second stint with Super Rugby's Chiefs after being a member of their 2012 championship side.
A return to rugby union could put Williams in the frame for a tilt at winning gold for New Zealand when rugby sevens makes its Olympic debut at the 2016 Rio Games, and Tietjens had little doubt the powerful centre could make the grade.
"Sonny Bill Williams would make the transition to sevens, there's no question about that," he said.
"Mainly because he fits the culture that we live. Not only is he a humble guy, but he's got a great work ethic.
"He works very hard, sets himself challenges and goals and goes out and gets them. He obviously has the work ethic, the skill-set, and is a real athlete. He could obviously be a very, very good player."
DISTINGUISHED CAREER
Williams has remained tight-lipped about his next career move, but Tietjens was confident the dual code international would be back with the Chiefs, who won a second successive Super Rugby title this year.
The coach would be quick to gauge his interest in his long-term plans, but suggested Williams would have to commit early to the sevens project to have any hope of going to Rio.
"If he's going to the (rugby union) World Cup in 2015, they'll have to give me an indication prior to the completion of that, that he'd like to have a crack at 2016," he said.
"And then once I have an indication then I suppose I can start building a squad.
"But also there will be some standards that I'll set around that.
"If (players) put their hands up and express an interest they'll have to play some (sevens) world series tournaments that can certainly confirm to me that they're good enough.
"For sure, we'll welcome any players in at any time but it is quite a demanding sport where you have to work exceptionally hard."
Williams' return would be another coup for New Zealand rugby after Benji Marshall signed a two-year deal to play for Super Rugby's Auckland Blues after a long and distinguished career in the NRL.
Former New Zealand rugby league captain Marshall has made no secret of his hopes to push for an All Blacks jersey for the 2015 World Cup and in the Olympic sevens.
Tietjens, who was preparing his team for sevens' season-opening world series tournament in the Gold Coast this weekend, said Marshall would also have what it takes to thrive in the Olympic format.
"I felt personally that it would be great for Benji Marshall to come to this tournament. It would have been great to have him involved because he's a great touch player," he said.
"I'm sure if he got the conditioning under his belt he'd be a very, very good sevens player."
(Editing by Patrick Johnston)
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