The Danish veteran fell to the ground in the 26th minute of the LIVESTRONG Sporting Park clash because a fan threw a bobblehead figurine of teammate Omar Bravo.
Nielsen required medical attention but returned to the pitch with a patch under his left eye much to the delight of the Kansas City fans.
"I did not see it coming," Nielsen said. "First I thought I walked into the post, then I realized I was a long way from the post.
"I had a crazy headache. I was like 'Oh my God, I need to lie down for a second. Then I saw I was bleeding and was just waiting for them to come and fix it so I could play again."
But the incident did not go unpunished as KC vice-president of communications Rob Thomson revealed.
"Two people from the same group in the third row both allegedly threw the head of bobbleheads, one of which struck Nielsen," he said. "Both were apprehended."