Hassell and Birmingham's Neil Danns were both shown red cards for their part in a scuffle that broke out after the final whistle.
The club examined video footage but any evidence was inconclusive.
Bobby will now miss three of the remaining four games with Southend, Crystal Palace and Leicester City but will be available for the final game at West Bromwich Albion on 6th May.
Meanwhile captain Paul Reid will return to the squad for the six-pointer at Roots Hall on Saturday.
Reid missed yesterday's win with a foot injury but will return to training this week. He could have a fight on his hands to get back into the starting line-up however after Antony Kay and Lewin Nyatanga played their part in keeping the clubs 9th clean sheet of the season.