In a bid to raise standards after a third-straight wooden spoon, Marshall made a pact with his players that they would return to pre-season training in good nick.
But Fainu, who was once touted as Daly Cherry-Evans long-term successor at Manly, was one of several players who rocked up to Concord in November either out of shape or unfit.
It meant Fainu had to front the Tigers' board as well as getting a rev-up from Marshall.
"That was a wake-up call, that I needed to wake up to myself (to prove) I want to play footy," Fainu told AAP.
"It was a big wake-up call to show that I need to get my body right.
"Benji gave it to me straight.
"In our end of the year meetings, Benji had told me where he wanted to be and I didn't show up to it.
"I didn't like seeing Benji get mad. He told me I could do better and I 100 per cent know I can.
"With Benji, it's like being with your older brother - you can talk to him and he will give it to you straight."
Fainu is all too aware that his indulgent off-season - he is quick to point out he is back to his playing weight of 88kg - could limit his involvement to start the season.
The 19-year-old spent most of his debut season in the halves but will now be expected to take a backseat to fellow teenager Lachlan Galvin as he looks set to partner Jarome Luai.
"I really want to play with 'Romey' (Luai) and that takes time for me to work hard and whenever Benji thinks I'm ready, I'll be ready," Fainu said.
"He (Luai) will bring a winning culture to this team and a competitive side to this team where he wants to win in every training session - and that makes us better.
"Me and Lachie have been working really hard and it'll be fair from Benji if he does pick me or Lachie.
"I'll play whatever position he wants me to play."
