Both players were substituted in the first-half on Sunday's 2-1 loss to Liverpool.
Speaking after the game, Lampard said: "Emerson is a similar injury to what he had, with a muscle.
"When you do that it means you've come back too soon. That's not a slight on Emerson, everyone wants to play this game, fair play. Those things shouldn't happen.
"Christensen is a bang on the knee which we will assess. The first thought is it's not that bad."