Zabaleta was the player who looked most in need of a rest at the end of last season, And then he had Copa America. Giving Sagna a run of games might also allow him to reach a higher level than last season, making it less of an issue when Zabaleta needs a break.
I dont think its as big a deal rotating the fullbacks as the cenrebacks. I thought it worked well when Zabaleta and Richards shared the position. But if we are to rotate them Sagna needs to play often enough to become comfortable in the team.