I think so far he's shown that he can come in and fill a spot in case of injury. Not sure he's too good to be left out of the team. While he's been defensively good for a small half, he'd probably be a liability at centre. Defending next to a half will put more pressure on his defence. He also goes low a lot which allows a fast play the ball, teams would just roll through his edge.Jury still out on Joash being a 5/8 for me so could slot into centre as too good a player to be left out of the team. Iongi is a 5/8 turned into a f/b so Fa'alongo would improve the team overall imo. All speculation of course unless actually happens.
Hopefully he has a huge offseason and plays well in the 6 next year, if not he's a good back up in case Iongi is not available

