British light-heavyweight Buatsi secured a victory yet unimpressive debut to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a tight ten-round bout at the Co-op Live Arena.
But Parker found success, delivering effective individual punches in a fight where the techniques failed to mesh, with constant grabbing disrupting the action.
Two judges judged it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my finest showing but what defines champions? They succeed," Buatsi commented in the arena as he was greeted with disapproval from the crowd.
The Londoner bounced back after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers believed the challenger, 31, deserved the win but the contender endured the second setback of his 28-bout professional journey.
The Champion Claims Victory in Slow Match
His career development has plateaued in recent times through contract problems and long spells of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now finds himself trying to realize his potential - maybe in what seems like a final roll of the dice - under recent support.
He opened tentatively, with neither fighter willing to be aggressive in a calculated initial rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after damaging his hand mid-fight, was participating in his second appearance at the higher weight class.
By the fight's midpoint, the underdog had kept it close, making life awkward for the favorite, who operated with a solid defense to barely take the better stanzas.
The spectators' acceptance decreased because of regular grabbing, with the challenger going down multiple occasions during clinches.
Parker finished the stronger, but multiple periods were hard to judge. Although, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Inspires All-Manchester Battle
In the supporting bout, Arthur defeated Bradley Rea by divided decision in a terrific local rivalry contest to secure the European light heavyweight championship.
The stadium fell silent for a profoundly moving memorial ahead of the contest as ten bells sounded in remembrance of Manchester's own Hatton, who succumbed 46 years old in recently.
It was the premier televised fight night in the urban center since the passing of Hatton. As the final chime ended, the quiet transitioned to a roar of "only one Ricky Hatton".
The fighting during the bout reflected the feeling. Arthur was sent to the canvas by a left hook flush on the jaw in the second round, rising shakily to his footing on the referee's count.
But the traditional red attire of Arthur soon came forward again. On the opposite side, the opponent - in rival City blue - was injured soon, heavy swelling forming under his face as the fighters exchanged punishing blows.
The challenger, 27, delivered effective strikes but the champion, 34, fought fire with fire with stinging uppercuts as the intensity hardly diminished over the full distance.
The victor barely secured the verdict with scoring tallies of 114-114, close margin and 115-112, but each competitor were persistent and courageous, everything their hero the legend embodied.
