Can I hit my head with a soccer ball ( during a match) 45 days after a FUE?

If the injury was severe enough to cause normal hair loss then it may affect your transplanted hairs.

At this point the grafts are completely incorporated into the scalp and you can do anything that you would have done had you not had a procedure.That includes heading a soccer ball!

As long as the area does not bleed from the hit or get burned ,than you won't cause a trauma. the first 10 days are the most critical time zone for a hair transplant operation. 

If you hit the ball and scored a goal = no problem. If you hit the ball and missed a goal = no problem but it is not good for your soccer team. Ha, ha.
In summary, no worries and your recipient area should be OK.
Keep in mind the FUExcision is a technique for donor harvesting ONLY.
Good luck.

You can use  your head to hit a ball for a sport without difficulty assuming that everything is healed at this time