Henry has hit career-best form over the past five weeks, averaging 26.2 possessions and two clearances across that stretch.
It's a huge improvement of his average of 12 disposals and 0.6 clearances per match across his 38-game career.
Reports have emerged that St Kilda are keen to lure the 21-year-old, who was snared with pick No.9 in the 2019 national draft and has huge potential.
When asked whether Ryan was a flight risk given he is still yet to sign a new deal, Longmuir replied: "He has to be, getting to this time of the season without him committing.
"In the end, the ball's in his court. The ball is well and truly in his court.
"I'm optimistic he'll stay. I see this as the best place for him to develop his footy and take his career forward."
Henry has been dropped to the WAFL on numerous occasions over the past four years.
Longmuir praised the way the speedy wingman has accepted each challenge and improved his consistency.
"I feel like as a coach I've given him a lot of opportunities along the way," Longmuir said.
"He's played 40 games across his four seasons. We've given him a lot of opportunities.
"There's been times when he's needed to go back and work on things we're asking him to work on.
"I think he's done that and embraced that really well, and that's got him to the point where he's played his most consistent over the last period of time."
Dockers captain Alex Pearce will play out the season despite an ongoing battle with a lower leg and foot injury.
Pearce has been unable to train for much of the season, and his performances have suffered at times because of it.
Fremantle (7-11) are almost certain to miss finals, but Pearce is determined to play out the final five games, starting with Saturday's clash with Geelong at GMHBA Stadium.
"He wants to keep pushing through," Longmuir said.
"And he wants to continue to build his footy and leadership and work on all aspects of his game and be there for his teammates.
"While his mindset is like that, we'll keep backing him 100 per cent
"I don't think he'll need surgery (at the end of the season). It will need a little bit of rest."
Ruckman Sean Darcy (ankle), midfielder Nat Fyfe (foot) and defender Brandon Walker (knee) have already been ruled out for the rest of the season.
Bailey Banfield (knee laceration), James Aish (quad) and Nathan O'Driscoll (calf) are all a chance to return against the Cats.
Defender Brennan Cox (ankle) is still one to two weeks away from returning.
