The Queensland Marroons star was taken from the field on a medicab just eight minutes from full-time at Aussie Stadium last night.
Flannery was a late inclusion for the Roosters after Adam Schubert pulled out due a knee injury.
Roosters club doctor John Orchard told AAP that Flannery will be out for a minimum of eight weeks, but a four-month stint on the sidelines was a possibility.
Meanwhile new Roosters recruit prop Chris Beattie was placed on report for a high tackle on Tigers hooker Robbie Farah.
The match itself was a low scoring affair as both teams looked to be underdone, the Roosters in particular bombing a number try scoring opportunities.
Pearce played the first half before Soward came on in the second half to score the match-winning try.
For the Wests Tigers, new half back John Morris was a stand out while forward Keith Galloway put in a strong first half display. Wests Tigers coach Tim Sheens stated that no injury worries came out of the match.
TIGERS 4 v ROOSTERS 10 (Foundation Cup) Scorers Tigers: T Tuiaki try. D Collis 0/1 goals. Scorers Roosters: J Williams try. J Soward 1/1, S Foley 0/1 goals. Aussie Stadium, Sydney, NSW 17 Feb 2007, 7.30pm HALF TIME: 4-all FULL TIME: Roosters 10-4 Crowd: 16,198