The most disappointing thing about the way Sonny Bill Williams left is that we didn't get a chance to say goodbye.

Just as with Andrew Johns, whose career ended suddenly after a collision with Knights teammate Adam Woolnough at training led to the diagnosis of his serious neck injury, nobody watching Williams's last match for the Bulldogs knew they were doing so.

And every time we see vision of him playing for Toulon or inevitably lining up with the All Blacks in the future, league fans will be cruelly reminded of what the game has lost.

The task facing the NRL is to ensure its competition, which had been universally accepted as the best of its kind in either rugby code, doesn't lose more talented stars.

But Williams can never be replaced. He wasn't just a once-in-a-generation player, Sonny Bill was a once-in-a-lifetime superstar.

Going to see Williams play was like watching the Johns brothers, Andrew and Matthew, at the height of their powers with the Knights, or Anthony Mundine and Nathan Blacklock at their razzle-dazzle best for the Dragons - except there was only one of him, and he was just as exciting to watch in defence as he was with the ball.

The Dogs may have only won five of the 13 games he played for them this season but at least when he was there they were in with a chance. They didn't win any of the four games he missed before his dramatic walkout on July 26, and they haven't won in the four weeks since.

Perhaps because he wanted to ensure no one could ever accuse him of not putting in even if he didn't want to be at the club, Williams played through injuries and was at his absolute wrecking-ball best every time he took the field - and not just for the Bulldogs. In May's Centenary Test, he was arguably the best player on the field despite being in a badly beaten New Zealand side.

While it is hard to accept the 23-year-old walked out on his teammates with seven rounds remaining, the signs had been there for some time that he was unhappy - both at the Bulldogs and with other aspects of his footballing life.

In mid-January, while Khoder Nasser was in New York with Anthony Mundine for the Felix Trinidad-Roy Jones jnr fight, Williams phoned and asked him to take over from Gavin Orr as his manager. The Bulldogs held talks with Nasser and Williams but obviously they never really resolved anything. The fact that Williams was prepared to pay $750,000 for a release from his contract with the Bulldogs shows how keen he was to get out of the club.

It is also an indication of how much money he must be earning to play rugby union for Toulon, and that is the area in which the NRL must act before the premiership's star power is eroded further. Of the eight players chosen by the NRL last year as the most marketable for the inaugural series of action dolls, three - Williams, Mark Gasnier and Craig Gower - have since joined French rugby union clubs.

For the likes of Greg Inglis, Karmichael Hunt, Israel Folau, Benji Marshall, Krisnan Inu and Jarryd Hayne, playing rugby in France, England, Ireland and even Italy or Japan has now become a genuine option that most are prepared to at least consider.

And if the NRL thinks no one else would be prepared to walk out on a contract, they should think again.

Sione Faumuina's departure from London Harlequins at the start of last season was almost identical to the way Williams quit the Bulldogs, with the former Warriors utility flying to Queensland without informing his club and joining the Cowboys, while Brian Carney simply retired after one training session last year with Gold Coast Titans before signing a lucrative rugby union contract in Ireland.

Few would be surprised if Penrith strike forward Frank Pritchard suddenly walked out in frustration after repeatedly having his requests for a release knocked back.