I'm not sure starting Pablo would do the team any good either...
The rotation right now is Price (ace), Buccholz (injured by AS break), Porcello (pitching like garbage atm), Kelly (has never pitched well for a full season) and Wright (never been a full time starter and is just a stand in for ERod who is hurt and who know when he'd pitch again). I'd say Shields is quite the upgrade at SP since he can consistently pitch 200 innings, which Porcello/Kelly/Buccholz/Wright/Rodriguez are unlikely to do.