Ullmark sustained a lower-body strain versus the Panthers on Wednesday and will be evaluated Thursday, Andrew Wilimek of the Senators' official site reports.
Ullmark had to leave Wednesday's game in the third period after going down in his crease. The 33-year-old likely wouldn't have started Thursday versus the Sharks anyway, but the question now is if he'll be able to serve as the backup. If not, expect the Senators to make a roster move to add Jonas Johansson from AHL Belleville.