Leicester director of rugby Richard Cockerill said after the match that it was too early to assess the damage but he has since admitted that the situation 'is not great'.
Both players limped off late on at Sixways and the Tigers are now awaiting further medical opinion on when they will be able to return to action.
But with England's 6 Nations opener against Scotland now just five weeks away, the news will be particularly worrying for new interim England boss Stuart Lancaster.
"I am just guessing at the moment until they have had their scans and we have received all the medical opinion, but it looks like they will be out for weeks rather than days," Cockerill told the Leicester Mercury.
"It looks like being a few weeks at best."
Lancasternames his Elite Player Squad on January 11 and Tuilagi was expected to be one of the first names on his teamsheet for the tournament opener at Murrayfield on February 4.
But although both Tuilagi and Deacon could still feature in Lancaster's 32-man party, they are unlikely to play in Leicester's crucial Heineken Cup clash with Ulster two days after the EPS announcement.
The January 13 encounter at Ravenhill is likely to decide Leicester's European fate this season but it would now be a major surprise if either player was fit to start in Belfast.