Although statically you could concieve of the fan 'sucking in' air, in flight their is a pressure rise in the diffuser section of the intake as the air slows down before going through the fan, statically the air has to be accelerated first.
This creates a momentum drag component on the engine due to the force required to slow the air down. Of course a large amount of the power required to overcome this is recovered as the air expands again through the nozzles.