Jacoby Ellsbury tweeted this a few minutes ago:
A subluxation is an incomplete or partial dislocation of the shoulder. The injury can be treated with rest, rehab and strengthening exercises.
Other athletes have taken 6-8 weeks to return. But further dislocations can require surgery to repair. As Ellsbury indicated, it's apparently not yet clear how long he will be out.
But obviously he will go on the disabled list today and the Red Sox will be without him for a while.