Brian McTaggart explains how the DL works in Spring Training in his new article.
Essentially, the earliest a player can be put on the DL is Friday. If a player appears in a game in which admission is charged after he is put on the DL, it affects that player's eligibility. So it's not like the Astros could put him on the DL on Saturday, have him play the exhibition games at Minute Maid against the Blue Jays, and then go back on the DL. That's why this weekend is so big for Lance: if he responds well to his rehab, he may not have to go on the DL. If it doesn't, and he goes on the DL, he'll miss the first four games of the season for sure.
Alberto Arias is also likely headed to the DL, but those two are the biggest DL risks as of now.
One guy who looks like he's pitching with a noodle arm is Tim Byrdak, but Brad Mills ain't scurred:
"No. 1, you're always kind of wondering, but at the same time he's really shown flashes of how he throws during the season. Looking back at his spring history, he hasn't had the best Spring Trainings in the past but really throws the ball well during the season. You don't want to overlook what's happening this spring, but if there's some adjustment that can be made, [pitching coach Brad Arnsberg] is making those adjustments the best he can. At the same time, you can't put all the stock in what' happening this spring."