An MRI required by the Ontario Athletic Commission 60 days prior to the bout revealed a damaged blood vessel, an injury Foster believes took place during a hard sparring session before the test.
"I was dealing with a really good boxer and he caught me with a left hook that rocked me a little bit," Foster said. "I hadn't been rocked like that very often. I've never been knocked down. My record says I've been knocked out before but I was not unconscious in that fight.
Foster's manager, Marc Fiore, forwarded the MRI results to UFC physician Dr. Jeff Davidson, who made the call to pull the fighter from the event at Toronto's Rogers Centre. Foster was informed Tuesday morning.
"When you hear something like that you think of the worst. I'm glad we got that scan," Fiore said. "Brian's a fighter. He's young and has that attitude that he'll fight no matter what. But it's not the right thing to do. I want him to be a father to his kids for a long time."
Foster will undergo an MRI in six weeks to determine whether or not he's fit to fight. The camp hopes the broken vessel will be completely healed and he will be cleared to compete. This is the first time Foster has dealt with a brain-trauma issue, Fiore said.
"He's not supposed to keep his blood pressure up," Fiore said. "He's supposed to do nothing for six weeks. My job as a manager, trainer and friend is to keep him away from anything that keeps his blood pressure up. Just enjoy life. Enjoy your kid.
"Brian is one of those guys, he's a leader. We say he's a captain of the team of fighters. He'll be here for the guys on the H.I.T. Squad roster. He's going to be here help coaching and keep these guys motivated. It's in his blood. If he's not fighting, he wants other guys to get ready."
Said Foster: "I plan to listen to the people that I trust most and take what they say into perspective."
Foster (15-5), who trains out of Granite City, Ill., at Finney's H.I.T. Squad, a facility formerly owned by UFC welterweight champion Matt Hughes, won his last two bouts in the Octagon. He's 3-2 in UFC competition.
"I understand I have stuff to look forward to," Foster said. "It doesn't bother me. I love to fight. I love to compete. My kids look up to me because of this. Everyone looks up to me to perform well. I just like making people happy. This is all negative. There's no positive to this situation here.
"It's positive that we caught it but negative that it happened. I have six weeks and I'm going to get another head scan. I'm 100 percent sure in my heart and my mind that I'm fine. I'll be back in the UFC and make a statement."
Though the UFC has not officially commented on the injury, the promotion intends to find a replacement to fight Pierson, a source said.
Josh Gross covers mixed martial arts for ESPN.com.