The Eels announced on Friday night they had secured Papalii's services for the next three years on a deal understood to be worth $1.5 million.

Papalii's departure would be a massive blow to the Raiders, given they identified him through their affiliation with the Souths Logan club in Brisbane and relocated his family to Canberra.

Tongue was at the helm of the Raiders in 2011 when Papalii burst into first-grade.

The 20-year-old has gone on to play 40 NRL games and is considered one of the most punishing hitters in the competition.

''It'd be devastating to lose him [because] the Raiders have put in so much time and effort to develop him and everybody holds him in such high regard,'' Tongue said.

''The way his footy's come on in the last couple of years, and especially in the last 12 months, you can just see he's going to be a real enforcer in the NRL for a number of years to come. You would've thought he would've liked to have stuck around, and hopefully he still might.''

The Raiders have the next three months to convince Papalii to have a change of heart.

While players can sign with rival clubs any time during the final year at their current club, the NRL does not register any contracts before round 13.

Penrith took advantage of that loophole in 2008 when it doubled its original offer to representative forward Luke Lewis, who backed out of a four-year deal with South Sydney.

That same year, the Raiders were burnt by the same clause when Nathan Smith reneged on joining the Green Machine and instead remained at the Panthers.

Papalii is understood to have told his Raiders teammates he wants to stay in Canberra, in a sign he could be convinced to change his mind.

Raiders winger Sandor Earl backed Papalii's move to join the Eels and said it was part of the modern game. ''Congrats to Big papa anyone who bags him obviously doesnt realise the limited time in the sport gotta make the most of it!!,'' Earl tweeted.

''Lookin forward to the year with him on the right side!! ''send him off with a premiership.''