Fekitoa signed a two-year deal with the Top 14 side, confirming the deal via his Instagram page last month.
The 24-Test player spoke for the first time on Thursday about his decision to leave, revealing a slip down the pecking order with the All Blacks was the main decision to leave.
"It was the hardest decision I have ever made," Fekitoa told reporters.
"I just started my career and even though 90 percent of fans and supporters disagree with it I believe it is the right to do that and I think it will make me a better player.