Fainga'anuku was a dynamic performer playing as a flanker in Friday night's 36-20 victory over the higher-ranked Auckland Blues in Christchurch.
It was a third successive start in the pack for the 26-year-old, whose 12 Tests for New Zealand have all been as an outside back.
Coach Rob Penney identified Fainga'anuku had the physicality and tackle-busting power to switch roles but admitted the change carried risk.
"You don't want to burden a player by putting them in a position that might compromise them, because there's a lot more brutality and physicality in the forwards," Penney told reporters.
"But he really wanted to do it and I'm just so pleased for the man.
"He's had three goes now and he's learning every week. He's adding value on both sides of the ball now and he's emptying his tank."
Fainga'anuku was a constant menace at the breakdown, winning several turnovers, and regularly broke tackles close to the ruck.
His skills honed in the backline were also evident.
The former Toulon player threw the last pass to set up the game's opening try to prop George Bower, who was playing his 100th match for the Crusaders.
Fainga'anuku then created two further tries through attacking kicks, finished by Jamie Hannah and Sevu Reece.
The Crusaders scored four of their six tries in the second half after trailing 13-12 at the interval.
Fainga'anuku admitted his body felt more battered after playing in the forwards than any match in the outside backs.
"It's a tough job and it's an awesome challenge that I enjoy waking up to," he said.
"I wouldn't say I'm comfortable but it always makes it easier when you've got a good forward pack like we do."
Victory lifted the Crusaders one place to fourth, with three rounds remaining until the play-offs.
They moved ahead of the Queensland Reds, who were beaten 31-21 by the second-placed Waikato Chiefs in Brisbane - a sixth straight win for the visitors.
The Blues' fourth loss of the year leaves them third heading into next week's match against the competition-leading Wellington Hurricanes.