^i read the transcript of the episode to see if Hasook actually packs a punch, to find that Bolin sucks at least as much as Hasook in that match. Yet Mako only jerkbends at Hasook about it. As far as Masook goes, i think most of that happened before the start of the series, and their quibble and subsequent no-show of Hasook was the tail end of that. especially considering how jealous Mako reacts when right after the breakup he finds Bolin with yet another girl in the locker room. a waterbender no less.
A failed Masook relationship would also contribute to Mako's "dating team members is a bad idea" attitude. Hasook used to be a good pro-bender, but his anger at Mako made him sloppier and sloppier. He intended to apologise, but by then it was too late- he found out he was replaced by the Avatar, and couldn't bring himself to return.
At Varrick's wedding he runs into Mako, and they become friends again.
Masook fits alongside those other totally canon pre-series relationships, Korra/ Howl and Asami/ Tahno.
While they catch up during the reception, Hasook reveals that after being replaced by the Avatar and quiting probending altogether, he met another waterbender who was also down on his luck and done with the leagues, they struck up an friendship, discovered they shared a passion for jazz music and decided to form an band together. Three years later, they're preforming at the Varricks' wedding and Hasook is now dating the former Wolf-Bat-turned-trombone player.
Mako and Asami then get to bond over the fact that both their exes have happily moved on with each other.