Pretty sure Lampard's main reason was James had been impressing and he decided to play him. Resting Azpi was the secondary reason - perhaps a diplomatic response of dropping him on the back of that 4-4 draw with Ajax.
Azpi's defensive performances over the last 2 months or so have generally been fine, this after a bad start. But of course, no one said anything. One not so good performance and suddenly people are getting their pitchforks out again. People thought he would struggle against Sterling at the weekend but he kept him quiet and did a solid defensive job but no one said anything. Was Azpi really the worst last night? Could make a case for Zouma or Christensen. The biggest question/problem, if anything, is why did Lampard suddenly change the entire back 4 last night for a key game? They played like strangers instead of a unit, made worse by the fact that they got no defensive protection whatsoever from the midfield.