I had a FUE hair transplant 45 days ago. There is bald patches on the donor area. Is this temporary? (photo)

This could represent shock loss of the donor area, which is relatively rare in my experience, but does occur. If so, this should start to recover in the next 3 months. If not, then this likely represents overharvesting of the donor area, which is irreversible but can be improved to some degree with scalp micro pigmentation

Your donor area will always look "thinner" proportionate to the number of hairs taken out after a FUE harvesting surgery.  The hairs will NOT grow back from the donor area (where it was taken). If it all goes to plan the donor hairs will grow in the area it was transplanted.

This is one of two things: either the donor was over harvested, in which the only viable option is SMP into the donor area- or this is temporary shock loss of donor hair, in which over the next six months or so the donor area will rebound and look much better. The operative report and possible immediate post-op photos will help determine which of the possibilities is most likely. Otherwise only time will tell.