Hi Rebecca,
I have no experience of hip replacements, but my immune system is weak (specifically low platelets and neutrophils).
For WBC (including neutrophils) you can be prescribed filgrastim injections (which can be done at home).
For platelets, an infusion of blood platelets from a blood donor can be done. That stop the risk of excessive bleeding (unfortunately the extra platelets don’t stay in one’s body for long).
Regards
Rabbit
PS I did see a study that eating dark chocolate boosts neutrophil counts.