"How concerned should the Angels be about Ervin Santana's sprained elbow?"
I hate being a downer just a week before the start of the season, but this is not good for the Angels. Santana is dealing with a sprained UCL, and while it's not enough to require surgery yet, he's not expected back until early June. Right now the plan is to let him rest, and hope that the ligament can scar up and allow Santana to pitch. Scar tissue is, at best, usually only about 70 percent as strong as the regular ligament, and though it's rare, if the scar tissue overlays the healthy ligament, he may be fine. This looks a lot like what happened to Francisco Liriano in '06 when he sprained the ligament, rested, and then it snapped in his first game back. The Angels will do everything they can to avoid that situation, but surgery may be inevitable.
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John Lackey had been one of the game's true mound horses until last year, when a triceps injury forced him to miss the first six weeks of the season. The good news is that once back, he showed no ill effects, but the bad news is that a recent forearm strain means he'll likely open the season on the DL. He's been shut down for an undetermined amount of time as the team awaits the full results of an MRI. If the problem persists, he could be looking at the same type of injury that knocked Andy Pettitte out for half of the year during his first season in Houston. If it's even worse, then you can start looking in Ben Sheets' direction. Right now there's not enough evidence to think it will match either of those scenarios, but that could change. By the way, it's a contract year for Lackey, and it doesn't look like he'll get a deal done for a while, so don't be surprised if he tries to push through this injury or others that might pop up during the year.