In case you missed it late last night, the diagnosis on Michael Pineda is a Grade I strain of the teres major muscle in his back. He'll be further evaluated by Dr. Ahmad today, but the early estimate from the team was an expected 3-4 weeks out and then rehab appearances after that to build his pitch count back up. Given how careful they were in managing his shoulder rehab, I anticipate the team will take the same approach here. We could be looking at 6-8 weeks when all is said and done.
As for the injury itself, it's less severe than a tear of the muscle, which is considered an uncommon injury by the American Journal of Sports Medicine. Generally, a tear of the teres major tendon takes 3 months to come back from, so a 3-4 week downtime estimate is encouraging given the potential worst case scenario. The muscle strain is limited to the back, and has no relation to Pineda's previous shoulder injury, although a strain in that area could and usually does manifest itself in upper arm pain and it would be bad for Pineda to alter his pitching mechanics to compensate for that and risk damage to the shoulder.
This injury is definitely not a good thing. Between the pine tar suspension and this, the Yankees are losing a key part of their rotation in a time where they can least afford to be without him. It's not as bad as it could have been though, and Pineda will be able to return to the mound this year.