According to Dan Roche on Twitter, Terry Francona has confirmed that Jacoby Ellsbury will be our starting left fielder in 2010. This also means Mike Cameron is in center field and Jeremy Hermida is the fourth outfielder.
Overall I think the move is fairly neutral as we discussed before here. His huge UZR numbers in left field are small sample size and moving Mike Cameron to left would probably get a similar defensive increase in run value.