The Canadian side are looking to bolster their squad due to a number of injuries and after suffering a surprise loss at Toulouse last week.
Fash, 23, had been in Hull’s 19-man squad for tonight’s game against Wakefield Trinity and was hoping to make his first Super League appearance of the season.
However, he is now set to feature across the city at Hull KR, who ‘host’ Toronto against Batley Bulldogs at KCOM Craven Park on Sunday.
Fash, who toured Papua New Guinea with the Knights last autumn, will join Black and Whites team-mate Jack Logan, who is also with the Championship outfit on loan.
“It’s great to give Brad the opportunity to go out to Toronto and get a good number of games to help him get some important match fitness,” said Hull coach Lee Radford.
“Competition for places in the squad is very high now, so the loan spell will be a good opportunity for Brad to go out and prove himself.
“He’s going to a Toronto side with plenty of quality, so I’m sure he’ll come back to us in good shape after playing a couple of games.”
Fash made 21 appearances for Hull last term, but has not featured so far this time around despite long-term injuries to Josh Bowden and Chris Green.
He has only played one senior game, a League 1 contest for Doncaster against Hunslet on February 24.
Danny Washbrook replaces him in the 19-man squad for this evening’s contest, but competition for forward places is set to intensify with ex-St Helens back-row Andre Savelio set to arrive in the UK tomorrow following his move from Brisbane Broncos to Hull.
Furthermore, Gareth Ellis, who recently came out of retirement, looks set to continue for the East Yorkshire club.
Toronto have released former Castleford Tigers prop Jake Emmitt, who is ready to rejoin Championship rivals Leigh Centurions.
Reigning Super League champions will look to arrest a four-matching losing run tonight against arch rivals Warrington Wolves in a reprise of last year’s Grand Final.
Wigan held clear-the-air talks after last week’s home defeat by bottom club Huddersfield.