What is the rationale for taking aspirin after hair implant?

It is best to contact your surgeon for post surgery care and instructions. Personalized medical treatment advice on the Internet without a good history and examination is inaccurate and may lead to confusion and misinformation.

Unless you have a pre-existing medical condition that requires you to take aspirin i cannot see any clinical reason why you should be taking aspirin following hair transplant surgery. 

I usually have my patients wait about a week before they take any aspirin products because they are blood thinners and I want the blood to be able to coagulate around the grafts for quicker healing.  You may need to ask your Doctor what is the reason he wants you to take it verses what you have read on the internet.