From the Lions' official store:

I guess if Ndamukong Suh is drafted by the Lions he will have to find another number to wear, as fellow Cornhusker Kyle Vanden Bosch has already claimed No. 93. 

The No. 93 jersey was not worn by any player last year since the Lions wanted to honor Corey Smith, whose life was taken in a boating tragedy a year or so ago.  Smith wore No. 93 and the Lions decided to not make it available for a year.  I think Vaden Bosch is a good player and a good person to bring it back onto the football field.