Flanagan was released by the Roosters and has been heavily linked to Canterbury.
But Gallen believes the Sharks would be silly not to make an attempt to bring Flanagan back to the club he spent his junior career at.
“I love the club and if there was a chance to bring a 21-year-old who has shown he can handle the NRL, but also has plenty of upside in his development, then they’d be crazy not to consider it,’’ Gallen told David Riccio from The Daily Telegraph.
“The one thing I know about Kyle is, he‘s willing to work harder than the person next to him to become a better player.”
The Sharks have been linked to a number of playmakers including Benji Marshall, but a move for Flanagan would signal an intent to build for the future and the now.
Johnson could potentially miss the first 10 games of next season as he recovers from an Achilles injury, while Matt Moylan is dealing with chronic hamstring issues.
Flanagan made 20 appearances and scored four tries for the Roosters in 2020.