With Sheffield taking fielding practice this week and this piece being posted, I fear the idea of starting Sheffield and putting Murphy at second will eventually gain traction. Last year, according to Minor League Splits, Murphy was -2 Runs/150 games at third. While he was +1 run at secondbase in limited playing time, he would more likely be between -2 and -5 at second in the majors. Given his offensive potential, that's not a bad deal. But who replaces him?
Maybe F-Mart in 2,000 years, but Gary Sheffield would be the popular candidate in the short term. The last time Sheffield played the field regularly was in 2005 for the Yankees, so he's probably not exactly sharp out there. Not to mention his -26.8 UZR that year.
The point being, giving Sheffield anything resembling a full-time job in the field would be disastrous. And moving Murphy to accommodate him would be full-blow defensive suicide.