The Ireland international, 34, returned to the club from Wakefield Trinity in the autumn, with a main aim of helping coach their outside backs and the academy side.
Grix always wanted to continue his playing career, too, and he trained with the first-team squad throughout the off-season but was not given a squad number or registered.
However, after Huddersfield lost England winger Jermaine McGillvary to a hamstring injury suffered in the friendly at Wakefield - and ongoing concerns over the fitness of other backs - head coach Simon Woolford has brought him into the 19-man squad for the visit of Salford Red Devils.
“Initially we were looking for Grixy as a back-up option for injuries later down the track,” he said.
“However, the reality is, for one reason or another, we need him in the 19 now.
"We’ve obviously lost Jerry and Leroy (Cudjoe) from the starting backline meaning a bit of a shuffle.“I just feel that Grixy’s vast experience will help us given who else we still have doubts on ahead of the Salford game.
"Training today has been hit by the frozen pitches so we couldn’t run as many guys as we wanted to and I’m going to give a couple until tomorrow afternoon to prove their fitness for the game.”
In an unusual move - no doubt pre-empting this decision - Woolford only initially named 18 players yesterday afternoon when the club had to submit its 19-man squad.
Halifax-born Grix scored 57 tries in 161 games for Huddersfield during his first spell with the club between 2010 and 2016.
He was integral in their historic 2013 League Leaders' Shield victory but left to spend the last two years on a second spell with Wakefield.
Grix hasn't played in any of Giants' pre-season games with his last match being Ireland's 40-8 loss at Wales in November.