The U.S. soccer legend had quite a unique look on his head during the Fox broadcast ... with a distinct triangle appearing behind his ear -- leading to a ton of speculation on social media.

During a hilarious text exchange with former LA Galaxy teammate Mike Magee, Donovan provided an explanation for his new cut ... admitting he underwent a hair transplant two weeks ago -- and was told it wouldn't show up on camera!!

Unfortunately for Donovan, it certainly did ... and the internet had plenty of jokes to throw around at his expense, with a user on X saying ... "Landon Donovan's barber is currently being detained by authorities."

Another user chimed in with ... "Did Landon Donovan literally run out of the barbers half way through a haircut because he was late for work?"