There is talk that now the Sox need to get an ace.
There are only about a dozen or so true aces in the majors (arguably), and James Shields is certainly not one of them (nor do I think Lester fits in the category, to be honest).
What the Red Sox need now is someone who can win in the postseason... I believe there is a difference.
For whatever reason, Shields has not got a great postseason record.
If the Red Sox believe one of Miley, Porcello, Kelly, or Masterson can win in the postseason, I don't think it is worth wasting the money to go get Shields.
For comparison, I don't think Hamels is an ace, either, but his postseason record is GS 13, W-L 7-4, ERA 3.09, WHIP 1.053, SO9/BB/9: 8.5/2.3. He has demonstrated he can win in the postseason. I'd take him if we can weasel him away from Amaro without including Bogaerts/Betts/Swihart.