British light heavyweight Buatsi secured a victory yet unimpressive debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a split decision in a tight 10-round bout at the boxing arena.
But Parker found success, connecting with powerful clean blows in a contest where the techniques failed to mesh, with frequent clinching marring the fight.
The scoring judges scored it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my finest performance but what defines champions? They win," Buatsi stated in the arena as he was met by boos from the spectators.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a twentieth professional win.
Various analysts thought the challenger, 31, should have been awarded the victory but the British boxer suffered the second loss of his 28-bout professional journey.
Buatsi Secures Victory in Dull Encounter
Buatsi's progress has stalled in the past few years through contract problems and long spells of inactivity.
Previously predicted as the next star of British boxing, he now is positioned trying to realize his potential - perhaps in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers willing to take risks in a cagey opening few rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after breaking his hand mid-fight, was making just his second fight at light heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for the favorite, who worked behind a solid defense to narrowly win the better stanzas.
The spectators' acceptance diminished amid regular grabbing, with the challenger going down various instances during clinches.
Parker closed well, but multiple periods were challenging to assess. However, former world champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the presence of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still seems difficult.
Hatton Memorial Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a terrific local rivalry fight to capture the European light heavyweight championship.
The venue fell silent for a profoundly moving memorial ahead of the contest as 10 bells rang in remembrance of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the first major televised fight night in the location since Hatton's death. As the final chime disappeared, the stillness transitioned to a shout of "only one Ricky Hatton".
The competition within the ring reflected the feeling. The eventual winner was sent to the canvas by a significant blow flush on the face in the second round, standing uncertainly to his feet on the count of eight.
But the traditional red shorts of Arthur soon advanced again. Across the ring, the opponent - in competitive blue - was damaged early, noticeable damage developing under his eye as the duo landed powerful shots.
Rea, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with powerful upward punches as the pace rarely dropped over twelve rounds.
Arthur edged the verdict with judges scorecards of even score, close margin and 115-112, but each competitor were persistent and courageous, all the qualities their inspiration Hatton stood for.
