One judge scored the Manchester bout a 95-95 draw, while the other two saw it as a 96-94 victory for Buatsi.
After suffering a first defeat five fights to drop his record to 27-4, Parker insisted the decision should have gone his way.
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He told DAZN: “Fuming. I won that fight easy. I think every single person at ringside said I won it.
“I won literally almost every round. If I outbox him and he can’t even touch me, I should be champion now and going on to bigger fights.”
Many ringside observers thought Parker had done enough to win the crappy contest.
“Everyone’s got eyes, so they can see the fight,” Parker added. “He can’t even a land a clean shot on me. I should be champion. I’m just pi**ed off.
“(I’d probably give him) one round, and that was early. The later rounds, he couldn’t even catch me. I was comfortable in there.
“I should be going on for bigger titles and bigger money fights. I’m trying to change my daughter’s life, and I get decisions like that.”
Olympic bronze medallist Buatsi (20-1) returned to winning ways after suffering the first defeat of his professional career to Callum Smith in February.
Buatsi said: “I must admit it wasn’t my best performance, but what do champions do? They win. Regardless of how it comes, you take it.”
‘You can’t be on the floor six times and claim you won’
The fight was somewhat spoilt by excessive holding throughout, which on several occasions resulted in Parker dropping from the clinches to the canvas.
The pair each felt that the other should have been penalised for their tactics.
Parker said: “When he’s pushed me down, what do you want me to do?
“I can’t keep lifting him up. He’s 14 stone or whatever, I’m going to waste my energy. If he’s pushing me down, the referee should probably say, ‘stop pushing him down.’ I wouldn’t keep going down to the floor then.
“A little bit scrappy at times, but if that’s the way you can win a fight and outbox him and jab his head off all night, he couldn’t even catch me with a shot.”
Buatsi, with whom the judges appeared to agree with, was unimpressed by Parker’s tactics.
“I don’t know what style that is,” Buatsi siad.
“You keep holding and going on the floor… you can’t be on the floor about six times and claim you won the fight.”
The controversial nature of the decision is likely to fuel calls for a rematch, but the lack of entertainment the fight produced could lead Buatsi’s new promoter Frank Warren to think twice about taking up such an option.
