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

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.

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 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

This is a case of having a over harvested donor area.  One thing you can do to camouflage the thinning is Scalp Micropigmentation.

The patch could be related to anagen effluvium after donor harvesting with FUEexcision technique, do not panic, it could be temporary and DO nothing yet. I will suggest you to wait 3-4 months for hair regrowth and contact your hair surgeon for an evaluation too. Good luck.

Without looking at the immediate post op procedure pictures it is difficult to tell whether this is simply an area of graft harvesting that has undergone shock loss or whether the area has been over-harvested. In the former scenario the grafts that have not been extracted should grow back and provide you coverage.  In the second scenario the bald patch is unfortunately permanent. You can further conceal this bald patch with scalp micropigmentation should you wish to do so.

This could be the result of over-harvesting of the donor area. If it does not reverse in 2-3 more months, it could be permanent.  We have treated many of these with Scalp Micropigmentation which is the only effective treatment for a permanent depletion of the donor area