UK light heavyweight Buatsi earned a win yet underwhelming start to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a split decision in a close ten-round contest at the boxing arena.
But Parker found success, connecting with powerful individual punches in a fight where the approaches never quite clicked, with constant grabbing spoiling the fight.
Two judges scored it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my finest display but what defines champions? They succeed," the victor commented in the arena as he was met by boos from the audience.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in February - to claim a 20th career victory.
Several boxing experts thought the challenger, 31, merited the victory but the British boxer experienced the second loss of his 28-match career.
Buatsi Claims Win in Uneventful Match
The fighter's advancement has stalled in the past few years through promotional disputes and considerable time of inactivity.
Previously predicted as the next star of the British fight scene, he now stands trying to realize his potential - possibly in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with neither fighter ready to overcommit in a cagey initial rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was competing in only his second appearance at light heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who operated with a protective stance to edge the more effective stanzas.
The crowd's patience diminished during constant clinching, with Parker going down multiple occasions during grappling.
Parker ended stronger, but multiple periods were hard to judge. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor advances, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - based on this performance - still appears challenging.
Hatton Memorial Motivates All-Manchester Fight
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a outstanding local rivalry contest to capture the continental championship.
The venue grew hushed for a extremely emotional memorial prior to the bout as ten bells tolled in remembrance of Manchester's own Ricky Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast match card in the location since the passing of Hatton. As the final chime faded, the stillness gave way to a chant of "there's just one Hatton".
The action within the ring matched the sentiment. Arthur was floored by a left hook flush on the chin in the initial stage, rising shakily to his position on the referee's count.
But the Manchester United-red equipment of the fighter soon came forward again. On the opposite side, Rea - in competitive blue - was marked up quickly, heavy swelling forming under his eye as the duo exchanged powerful shots.
The challenger, 27, connected with strong blows but Arthur, 34, matched the intensity with stinging uppercuts as the intensity hardly diminished over the full distance.
Arthur edged the decision with judges scorecards of even score, close margin and wider gap, but each competitor were relentless and full of heart, everything their idol Hatton represented.
