UK light-heavyweight Buatsi secured a victory yet disappointing debut to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a competitive ten-round contest at the Co-op Live Arena.
But Parker found success, delivering effective individual punches in a contest where the approaches failed to mesh, with frequent clinching disrupting the contest.
Two judges had it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my finest display but how do champions respond? They prevail," the victor commented in the ringside as he was greeted with boos from the spectators.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in last winter - to claim a 20th career victory.
Several boxing experts believed Parker, thirty-one, should have been awarded the win but the Derbyshire fighter experienced the second setback of his 28-bout professional journey.
The Champion Claims Triumph in Dull Affair
The fighter's advancement has stalled in the past few years through promotional disputes and long spells of inactivity.
Previously predicted as the future of UK boxing, he now is positioned trying to achieve expectations - maybe in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers prepared to be aggressive in a cagey early stages.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was competing in only his second bout at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, causing problems for the favorite, who worked behind a protective stance to narrowly win the more effective stanzas.
The audience's tolerance diminished during frequent holding, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker closed well, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Russian star Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Tribute Motivates Local Rivalry Contest
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a terrific Manchester vs Manchester contest to secure the European light heavyweight belt.
The arena fell silent for a extremely emotional homage ahead of the contest as 10 bells rang in honor of the city's native Hatton, who passed away 46 years old in recently.
It was the first major televised fight night in the urban center since the passing of Hatton. As the concluding sound faded, the stillness gave way to a chant of "there's just one Ricky Hatton".
The action within the ring matched the feeling. The eventual winner was floored by a powerful punch directly to the chin in the early round, rising shakily to his position on the eight count.
But the Manchester United-red attire of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was marked up quickly, noticeable damage developing under his face as the pair traded hard punches.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the pace seldom decreased over 12 rounds.
Arthur narrowly won the verdict with judges scorecards of even score, close margin and clearer margin, but each competitor were relentless and courageous, everything their idol the legend stood for.
