Madcap Horschel, 20, is nicknamed Steve-O by his own team mates because he is a dead ringer for the "Jackass" TV clown.
And he lived up to his nickname at Royal County Down with a loud-mouth performance that included sprinting across the 14th green to scream "Yeah, baby" at his ball during the foursomes.
The Holywood ace crushed the American 4&2 to gain revenge for his 18th hole loss to him in Saturday's singles, when he missed a three footer on the last.
He was also annoyed by Horschel's behaviour in that 2&1 foursomes defeat that featured Ireland's Jonny Caldwell and American Rickie Fowler.
"Apart from what happened on the 14th, he was just very loud in the foursomes this morning as well. He wasn't a nice guy to be around.
"I definitely wasn't going to let him beat me twice. I was really pumped up to beat him. That is just the way it is.
"He seems to be okay off the course but on the course he was using gamesmanship and stuff and it was just ridiculous.
"If I tried to slow him down a bit he would struggle and I would get the crowd on my side, which he obviously didn't like.
"I got up on him and I probably should have beaten him yesterday as well. That is the way it goes. I definitely wasn't going to let him beat me twice.
"He is just very loud and I just knuckled down and didn't really speak to him for the rest of the round."
McIlroy will remain an amateur when he tees it up in the European Tour Qualifying School at the Oxfordshire tomorrow.
But he was delighted that he put off turning professional until after the Walker Cup.
He said: "It was worthwhile waiting for this. It is tough to lose but some of the guys will be back in America trying to win this back.
"The foursomes gave us too much to do in the afternoon. I can't wait to turn professional and I go to the Oxfordshire on Tuesday for PQ1
"If I can finish in the top 30 I won't have to worry about the Tour School until November and I have a few European Tour invites between now and then.
"Getting a tour card would put a smile back on my face. It has been a great year for me. I have played some great golf but I am slightly sad that it had to end this way.
"It has been a great amateur career for me and hopefully I can go on to achieve bigger and better things in the professional game."