Reid is in huge doubt for Friday night's clash with Adelaide following a hit to the "nether regions" during the opening quarter of last week's loss to Melbourne.
Longmuir said although Reid may not feature against the Crows, it's not an injury that will hinder him for long.
"We're monitoring it," Longmuir said.
"It's not groin soreness. From what you've said, maybe you're alluding to the fact he's got OP (osteitis pubis) or something like that. But it's not that.
"But yeah, he's in a bit of doubt."
Reid is yet to miss a game since making his AFL debut in round one last year, with the 2025 Rising Star winner's hot form putting him on track for All-Australian selection.
The Dockers (18-3) will secure the minor premiership with victory over fifth-placed Adelaide (14-7).
One selection call that is becoming harder by the week for Longmuir is the back-up ruck role.
Sean Darcy started the season as the preferred option to back up Luke Jackson.
But when Darcy went down with concussion and then a calf injury, it opened the door for Mason Cox to try to solidify his spot.
Cox has had a mix of strong and indifferent games during his 14 matches this season.
And with Darcy dominating in the WAFL in recent weeks, there is no guarantee Cox will keep his spot against the Crows.
"Sean's putting pressure on and he's playing really well," Longmuir said.
"His last two weeks, he's worked on the things we wanted him to work on.
"We'll weigh up that form against Mason's form."
Darcy is contracted until the end of 2030 and Longmuir said there were no plans for the 28-year-old to leave the Dockers, despite losing his spot as the No.1 or No.2 ruckman.
"I see Sean playing a big part in us going forward," Longmuir said.
Fremantle skipper Alex Pearce is set to return against Adelaide after being managed last week.
Pearce is signed with Fremantle until the end of 2027 and it's understood the Tasmanian isn't considering a move to the Devils in time for their inaugural 2028 season.
"It doesn't surprise me at all," Longmuir said of Pearce's commitment to the Dockers.
"I see his commitment day in, day out here. I see how he leads the club, and how passionate he is about the football club and his teammates."
