Gallen left the field in the 54th minute and took no further part in the match.
A full report on the extent of the injury is yet to be confirmed but indications are that Gallen may miss the opening round of the NRL premiership when the Sharks clash with Penrith on March 17.
The news of Gallen's injury follows reports of Bulldogs winger Matt Utai limping off in last night's trial in Wollongong, and Roosters star Chris Flannery being sidelined four months after suffering a broken leg last week.
It was a gutsy win for the Sharks which provided new coach Ricky Stuart with his first victory at the club. After being down 4-0 at half time, the Sharks came back to score four second-half tries, giving the club some cause for celebration following their record 10-game losing streak in 2006.
Brett Kimmorley's kicking game played a big part in the comeback, the veteran half back setting up two tries in seven minutes for Misi Taulapapa and Luke Covell. Brett Kearney and Fraser Anderson also crossed.
To put the trial in perspective, the Rabbitohs were without Roy Asotasi, David Kidwell, Nigel Vagana, Jeremy Smith, David Peachey and Nathan Merrit. The Sharks were without fullback David Simmons and five-eighth Adam Dykes was a late withdrawal with a shoulder injury.
Interchange specialist Dean Widders gave Rabbitohs fans something to cheer about with the former Eels star adding plenty of spark to the attack. Starting in the back row, Widders set up the Rabbitohs only try with a cut-out pass to winger Paul Mellor. Halfback Joe Williams was unsuccessful with the sideline conversion attempt.
Both sides had opportunities to post further points but the scores remained unchanged for the remainder of the first half.
The second half was hampered by thunderstorms, the greasy conditions making ball control difficult for both teams. The Sharks adjusted better to the conditions with Kimmorley's kicking boot taking charge of the proceedings
RABBITOHS 4 v SHARKS 22 Scorers Rabbitohs: P Mellor try. Scorers Sharks: B Kearney, L Covell, F Anderson, M Taulapapa tries. L Covell 3 goals. Central Coast Blue Tongue Stadium, Gosford, NSW 7.30pm Referee: Jarred Maxwell. HALF TIME: Rabbitohs 4-0 FULL TIME: Sharks 22-4