Gallen remains undecided whether 2018 will be his last at the top level, having re-signed on a one-year deal with the Sharks at the end of last year that will take him past his 37th birthday.

One of the finest players in the club's history, Gallen will forever be remembered as the captain who broke the Sharks' 50-year premiership drought in 2016 after enduring a number of difficult years at the club.

"I think the next two or three months are going to be really important to Gal with how he's feeling and how his form is," Flanagan said.

"Whether he's knocking on the top of the tree in all his statistics and challenging to be the best player in his position ... But we won't know until halfway through the season."

Gallen's position is no different to what it was a year ago, when he was also considering whether 2017 would be his last year in the NRL.

However, the competition's oldest player responded with one of the most consistent years of his career, named the Dally M Lock of the year and rewarded with Cronulla's player-of-the-year honour.

"I thought it was a super effort, he could have dropped his guard and said I've had a great career and won the club its first premiership," Flanagan said.

"He's one of the toughest and most competitive people I've ever known and he just loves it."

Gallen will enter Cronulla's season-opener against North Queensland in Townsville on Friday night without a trial match under his belt, after minor back soreness during the summer.

But Flanagan has noticed a significant rise in the 36-year-old this week.

Crucial to his late-career resurgence, Gallen said, was his decision to give up representative football at the start of last year.

The 304-game NRL veteran played every match of the Sharks' season last year for the first time since 2005, after ending his NSW State of Origin career in 2016.

"It was a major thing I think," Gallen said.

"I really noticed that when I played the City-Country game. The City-Country game was supposed to be a week off and I played it, which I'm glad I did.

"And I noticed probably two-three weeks post that game I was struggling. The body wasn't where it needed to be.

"And it probably wasn't until the week off we had about a month after it I was able to get back into the swing of things.".

Australian Associated Press