Because Messina was waived with the injured designation, there are a few different possibilities for what could happen next. First things first, another team could decide to claim him, which is what the Lions did with Bill Nagy last summer. If Messina clears waivers, the Lions could decide to put him on injured reserve. In this situation, they would also have the option to simply release him with an injury settlement, making him a free agent in the process.
Messina played against the New York Jets on Friday, but he missed five practices earlier in training camp because of a hamstring injury. Clearly it's still bugging him given the move the Lions made on Sunday.
I'd honestly be a little surprised if Messina doesn't end up on IR. I say that simply because Messina was quite good in last year's preseason, and he spent 2012 on the practice squad. Perhaps the Lions like some of their other young linebackers more, but Messina flashed some potential for Detroit.