A report emerged during Fremantle's pre-season win over Melbourne on Sunday that Jackson would be open to a move back to Victoria, despite being contracted with the Dockers until 2029.
Jackson won a premiership at the Demons in 2021, but his 52-game stint at Melbourne came to an end a year later after securing a blockbuster trade to Fremantle.
Longmuir couldn't have been more emphatic when asked about the speculation linking Jackson with a move back over east.
"Not a concern. Nothing I've seen of Luke throughout the pre-season has shown me he wants to leave the club," Longmuir said.
"Someone from the club has spoken to him already and he's reiterated that he doesn't want to go anywhere and he's never been happier.
"We'll leave it at that, I reckon."
Jackson looms as a key figure in Fremantle's push for premiership success this year.
The 23-year-old is set to spend the majority of the season as a forward, with Liam Reidy to shoulder the ruck duties while Sean Darcy (knee/ankle) recovers.
Jackson's ability to play in attack, the ruck or as a big-bodied midfielder has earned him the label of a football unicorn.
The 199cm star showcased his skills in Sunday's 19-point pre-season win - 16.13 (109) to 11.24 (90) - over the Demons, kicking two goals from 18 disposals.
He also tallied nine hit-outs during a short cameo in the ruck.
Longmuir kept his cards close to his chest when asked what role Jackson would be playing to start the season, which kicks off for Fremantle against Geelong at GMHBA Stadium on March 15.
"It's hard to say he's going to do this, this and this, because he is so flexible and we can use him in a lot of different ways," Longmuir said.
"In round one, that might be in the ruck, but it might not be. He might be playing forward, mid, in the ruck.
"Everyone sees what he does around the ball, and everyone assumes he's a midfielder every time he's around the ball.
"But a lot of the good work he does up the ground is when he's actually forward, because he's such a good aerial player.
"And then once the ball hits the deck, he becomes such a good ground-level player.
"He's vital wherever he plays. He really does help out Jye (Amiss) and JT (Josh Treacy) up forward to give us that third tall up there."
New recruit Shai Bolton kicked two goals from 13 disposals against the Demons.
"It was his first competitive game with us ... and he's coming off that little calf issue," Longmuir said.
"He probably felt the conditions and the heat and the fatigue a little bit more than others.
"I thought his best was very good, and there was a little bit of rust in other areas of the game."
