The fight was about to go to a referee decision when Ukrainian Sirenko secured the victory by knockout with the final punch of the match.
After a high-energy opening round, the fight was more disciplined until the Ukrainian's decisive combination, according to UFC.com.
Wallin lost consciousness in connection with the knockout, but was able to get back up after a while. He was taken to hospital after the match for a check-up.
"He's okay," assistant coach Travia Pierce writes to Sportbladet.
Zuffa is, among others, UFC chief Dana White's venture into boxing. The match was part of a gala at Madison Square Garden in New York.
