The fight was on Friday in Maryland. Dadashev was 28 years old.
"Top Rank is devastated to report that Maxim Dadashev passed away earlier this morning due to injuries sustained during last Friday's bout," Dadashev's promoter, Top Rank, said in a statement.
"Maxim was a talented fighter inside the ring and a loving husband and father outside the ropes. Our thoughts and prayers are with his family and friends."
"Maxim was a terrific young man," added Top Rank founder and CEO Bob Arum. "We are all saddened and affected by his untimely death."
Dadashev and Matias fought at MGM National Harbor in Oxon Hill, Maryland on Friday night. The winner of the fight would've been the mandatory challenger to Josh Taylor for his title belt.
Dadashev's trainer Buddy McGirt stopped the fight after the 11th round.
Matias was dominating Dadashev and was ahead 109-100, 108-101 and 107-102 on the scorecards at the time of stoppage, according to ESPN's Dan Rafael. In reviewing footage from the fight, McGirt could be seen and heard pleading with Dadashev to stop the fight, noting he had taken a lot of hits.
Dadashev had to be helped out of the ring, and he started to vomit and lost consciousness before making it to the dressing room, ESPN reported. He was taken to UM Prince George's Hospital Center in Cheverly, Maryland where he underwent emergency brain surgery for a subdural hematoma, which is bleeding in the brain, the network said.
It added that Dadashev had to be placed into a medically induced coma after the operation in order to allow the brain swelling to subside.
"As you can imagine the news this morning has devastated us all," McGirt wrote in a message to CNN. "Out of respect for his family I will refrain from making any comment at this time other than to say Maxim was and always will be a phenomenal young man and boxer."
Dadashev originally hails from St. Petersburg, Russia but was based out of Oxnard, California. He had a professional boxing record of 13 wins and one loss, with 10 of his wins coming by knockout, according to Top Rank.
The Russian Boxing Federation has announced that they have opened an investigation into the matter.