The decision to let Delmas go simply came down to his salary. He was set to have a $5.5 million base salary in 2014, and he was also due a $500,000 roster bonus. By cutting him now, the Lions have freed up $6 million in cap space. Add that to the $5.5 million they saved by cutting Burleson earlier and the Lions created a total of $11.5 million in cap space with these two moves alone.
Delmas joined the Lions in 2009 as a second-round pick. Injuries were an issue throughout his first four seasons in the NFL, and he ended up signing an incentive-laden contract with Detroit last offseason as a result. He finally made it through all 16 games, and he did make a lot of good plays as the emotional leader of the defense. However, his production wasn't worth the $6 million the Lions were set to owe him this year, so it's no surprise that he was released.
With the Lions parting ways with Delmas, safety is now one of the Lions' top needs this offseason. Don Carey could always start alongside Glover Quin, but it'd be better to keep Carey in a backup role and bring in someone with a bit more talent to replace Delmas in the starting lineup.
UPDATE: The Lions reportedly want to re-sign Delmas at a reduced salary.