British light heavyweight Buatsi secured a victory yet unimpressive debut to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a majority decision in a close ten-round bout at the Manchester venue.
But the challenger landed significant shots, landing some telling single shots in a match where the styles failed to mesh, with frequent clinching spoiling the contest.
Two judges judged it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my best showing but how do champions respond? They prevail," Buatsi stated in the ring as he was greeted with negative reactions from the spectators.
The Londoner got back to winning after his first professional defeat - an epic battle with his previous opponent in February - to secure a 20th pro success.
Various analysts believed Parker, 31, merited the victory but the contender endured the second setback of his 28-fight professional journey.
Buatsi Claims Victory in Slow Affair
His career development has plateaued in the past few years through promotional disputes and considerable time of inactivity.
Previously predicted as the future of British boxing, he now stands trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under fresh management.
He started carefully, with neither fighter willing to take risks in a cagey opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second appearance at light heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for Buatsi, who operated with a tight guard to narrowly win the better rounds.
The crowd's patience diminished because of constant clinching, with Parker slipping to the canvas several times during clinches.
The underdog closed well, but several stanzas were hard to judge. Nevertheless, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
The victor moves on, but with the likes of heavy hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Tribute Inspires All-Manchester Fight
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific all-Manchester battle to win the European light heavyweight belt.
The arena fell silent for a deeply poignant tribute ahead of the contest as 10 bells tolled in memory of Manchester's own Ricky Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast match card in the location since Hatton's death. As the concluding sound ended, the stillness gave way to a roar of "there's only one Ricky Hatton".
The competition during the bout matched the sentiment. The eventual winner was sent to the canvas by a powerful punch squarely on the jaw in the early round, rising shakily to his position on the eight count.
But the red-colored attire of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was marked up early, noticeable damage forming under his eye as the fighters landed punishing blows.
Rea, 27, landed power punches but Arthur, thirty-four, matched the intensity with effective uppercuts as the pace rarely dropped over twelve rounds.
The victor edged the result with judges scorecards of 114-114, 115-113 and clearer margin, but the two athletes were persistent and brave, exactly what their hero the legend represented.
