The Blues midfielder hobbled off the Hillsborough pitch with a hamstring injury.
Dunn was sidelined for most of the second half of last season but following a summer of extensive training it was thought he was ready for the new season which begins in just over three weeks.
However, fears are that Dunn has torn the hamstring again and if this is the case then the August 14th date against Portsmouth would seem unlikely for Dunn to make a return to Premiership action.Have Your Say At The Messageboard