British light heavyweight Buatsi secured a victory yet unimpressive debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a close ten-round fight at the Manchester venue.
But Parker found success, landing some telling individual punches in a contest where the styles didn't complement each other, with constant grabbing disrupting the contest.
Two judges judged it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my best performance but what defines champions? They prevail," Buatsi commented in the ringside as he was met by boos from the spectators.
The British fighter got back to winning after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Some ringside observers felt Parker, thirty-one, should have been awarded the win but the British boxer endured the second loss of his 28-fight career.
The Champion Earns Triumph in Uneventful Affair
Buatsi's progress has stalled in recent times through management issues and long spells of lack of fights.
Earlier forecast as the future of British boxing, he now stands trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers willing to take risks in a calculated early stages.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was competing in only his second fight at the higher weight class.
By the middle rounds, the underdog had made it competitive, causing problems for the favorite, who worked behind a tight guard to narrowly win the more effective rounds.
The crowd's patience decreased because of regular grabbing, with Parker going down various instances during close exchanges.
The underdog ended stronger, but several stanzas were difficult to score. However, former world champion Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just one round.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Hatton Memorial Motivates All-Manchester Fight
In the supporting bout, Arthur defeated Rea by divided decision in a terrific all-Manchester fight to capture the European light heavyweight title.
The stadium grew hushed for a deeply poignant homage before the fight as ten bells sounded in remembrance of the city's native Ricky Hatton, who died aged 46 in recently.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the final chime faded, the quiet gave way to a roar of "there's only one Hatton".
The competition during the bout equaled the emotion. The eventual winner was sent to the canvas by a powerful punch directly to the face in the early round, getting up unsteadily to his footing on the referee's count.
But the traditional red shorts of the fighter soon advanced again. On the opposite side, Rea - in opposing blue - was marked up quickly, significant bruising appearing under his eye as the pair exchanged hard punches.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the action hardly diminished over the full distance.
The victor barely secured the result with official scores of even score, narrow lead and wider gap, but both fighters were relentless and brave, all the qualities their idol the legend represented.
