UK 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 medal winner's punch output likely gained him a judges' verdict in a tight 10-round contest at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a fight where the techniques never quite clicked, with constant grabbing spoiling the action.
The scoring judges had it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my finest performance but what defines champions? They prevail," the victor said in the ring as he was greeted with negative reactions from the audience.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in February - to claim a 20th career victory.
Several boxing experts thought the challenger, 31, deserved the decision but the contender endured the second setback of his 28-match professional journey.
The Champion Claims Victory in Dull Affair
His career development has plateaued in the past few years through management issues and extended periods of inactivity.
Previously predicted as the next star of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under fresh management.
He started carefully, with both competitors willing to be aggressive in a calculated early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after breaking his hand during the bout, was competing in only his second appearance at light heavyweight.
By the halfway stage, the underdog had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to edge the better periods.
The audience's tolerance wore thin because of frequent holding, with the challenger going down various instances during close exchanges.
The underdog closed well, but multiple periods were hard to judge. Although, ex-champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
Buatsi moves on, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Homage Inspires All-Manchester Contest
In the secondary fight, Arthur overcame Bradley Rea by split-decision in a outstanding all-Manchester contest to capture the European light heavyweight belt.
The venue became quiet for a profoundly moving memorial ahead of the contest as 10 bells rang in remembrance of the city's native Ricky Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the concluding sound disappeared, the quiet transitioned to a chant of "there's just one Hatton".
The competition during the bout reflected the sentiment. Arthur was floored by a powerful punch directly to the face in the early round, getting up unsteadily to his position on the eight count.
But the red-colored attire of the fighter soon advanced again. Across the ring, the opponent - in competitive blue - was injured soon, significant bruising forming under his face as the pair traded hard punches.
Rea, twenty-seven, landed power punches but Arthur, 34, matched the intensity with powerful upward punches as the action hardly diminished over 12 rounds.
Arthur edged the verdict with judges scorecards of 114-114, narrow lead and clearer margin, but both fighters were relentless and brave, all the qualities their idol the legend stood for.
