McGillvary, 30, pulled a hamstring in Tuesday’s friendly at Wakefield Trinity.
Huddersfield’s Super League opener against Salford Red Devils looms next Friday but head coach Simon Woolford fears he could miss up to three months action.
As they await details on the severity of the injury, the Australian admitted being unsure about potential options for a replacement.
Winger Darnell McIntosh was the club’s top-scorer last season but has switched to full-back for 2019 and been impressing in that position during pre-season.
Huddersfield also have talented twin brothers Louis and Innes Senior to call upon but the versatile England Academy internationals are still relatively inexperienced teenagers.
One option, then, is to bring veteran Ireland full-back Grix back into the side to free McIntosh up to return to the flank.
Grix, who won the 2013 League Leaders’ Shield with Huddersfield during a six-year stint, rejoined from Wakefield in November as part of the deal that saw Danny Brough go the other way.
The 34-year-old was primarily recruited to start his coaching career, helping the outside backs such as McIntosh and the Senior brothers in particular.
Indeed, he has not been given a squad number for this season but last night told The Yorkshire Post: “I signed with the view that I’d be playing at some point.
“I wouldn’t have come here if I thought I was not playing again,” he said. “I feel good and feel I have plenty of good football left in me; only I’ll decide when I do retire.
“I’ve done a full pre-season’s training as normal and only played half the games last year (at Wakefield) so the body’s in good condition in that regard.
“But as a coach, the main reason was to help the young guys here – there’s a couple of young full-backs – and with me knowing my way around the club, hopefully having the respect for what I’ve done, it’s a great chance for me.
“I’m coaching the academy, too, and assisting Simon and Robbo (Luke Robinson), so it’s already a hectic schedule.
“I’m not registered but, if needed, I’m available. Admittedly, no one would have imagined it to happen as early as round one but if Simon wants me to I’m here.”