Santiago has played most of his 11 year career with the Tigers. A career .243/.311/.330 hitter, Santiago is best known for his work in the field. In 2013 he was especially good playing third base and shortstop, which is the need that the Reds have identified for their bench.
Reds GM had the following to say about the move (quotes courtesy of MLB.com):
We had identified three or four guys, but we liked him a lot. I’ve liked him for a while. I tried to sign him a couple of years ago before he signed his last contract with Detroit. He’s a solid defensive player at second base, shortstop and third base. He’s a switch hitter that runs well. He’ll put the ball in play. His numbers aren’t great but he handles the bat well enough that he’s not going to be a dead out. He’s another good character guy and a good guy for the clubhouse.”
That’s not exactly a ringing endorsement of his bat: “he won’t be a dead out,” meaning, he’s not the worst hitter in all of baseball or anything like that. These types of versatile players have value for NL teams, especially those with aspirations of making the playoffs. Santiago could be a solid pickup for the Reds.