It looks like Alvarez will being seeing time at middle relief. I would say Lackey, but he's gotten beaten up pretty bad in Spring training. I myself just picked up Jennings today (I traded Lackey a week back in a package deal for Pedro), but I'm not confident in that enough to give a solid recommendation.
I would say it would be between Jennings and Ainsworth, but I'd be interested in hearing what others have to say
"The hardest thing to do in baseball is to hit a round baseball with a round bat, squarely." -- Ted Williams