Young hurt his right knee while being tackled to the ground midway through the third quarter of the 14.15 (99) to 14.6 (90) win at Optus Stadium.
The 55,201-strong crowd groaned in unison when Young got up and could barely put any weight on his right leg after the tackle.
The 25-year-old gingerly limped from the field before undergoing a thorough assessment of his knee.
Young was eventually given the all clear to return, finishing the game with 15 disposals and three clearances.
Despite the quiet performance, the fact Young was able to play out the match is a big win for Fremantle following the midfielder's string of injury setbacks over the past 18 months.
"There was a little bit of worry that it might have been a bit of a twisting mechanism with his knee," Longmuir said.
"But his knee checks out well. There's no swelling at the moment, so it looks like we've dodged a bullet there.
"It looks like there was just a contact injury with the ground, so that's what we're hoping."
With Young not having his usual major impact, Andrew Brayshaw and Murphy Reid stepped up big time in the midfield.
Brayshaw had 16 possessions alone in the third quarter on the way to 35 for the match, while Reid continued his meteoric rise with 27 disposals, eight score involvements, 412m gained and one goal.
Reid, last year's Rising Star winner, has shown no signs of the so-called 'second-year blues', and Longmuir knows exactly why.
"For me, a real basic view of second-year blues is the fact that you don't handle your first off-season well," Longmuir said of the 19-year-old.
"Because you come back and you're behind the eight ball. But it wasn't the case with him.Â
"It felt like we set him up well in the off-season. He came back fitter, stronger, hungrier, and launched into pre-season.
"That sets up your second season, because realistically you should improve in your second year, shouldn't you?"
Reid played mostly as a half-forward in his debut season, but has been handed more midfield responsibilities this year and is relishing the role.
"We've clearly added layers and levers he can pull in his role. He's not just a half-forward anymore, he plays on ball," Longmuir said.
"His work rate is second to none. He loves chasing the pill around and he finds it and uses it well. He's been super."
Fremantle (13-1) sit a game clear on top of the ladder following their club-record 13-match winning run, and will start as favourites when they host Gold Coast on Sunday next week.
