UK light-heavyweight Buatsi secured a victory yet disappointing debut to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a split decision in a competitive 10-round contest at the Co-op Live Arena.
But Parker found success, delivering effective individual punches in a match where the techniques never quite clicked, with frequent clinching spoiling the contest.
Two judges judged it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my finest performance but what do champions do? They win," the victor commented in the arena as he was greeted with negative reactions from the crowd.
The Londoner got back to winning after his first professional defeat - an memorable fight with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers believed Parker, 31, merited the decision but the Derbyshire fighter suffered the second setback of his 28-bout professional journey.
The Champion Claims Triumph in Slow Affair
The fighter's advancement has slowed in the past few years through management issues and extended periods of inactivity.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to realize his potential - possibly in what seems like a final roll of the dice - under recent support.
He started carefully, with the two boxers prepared to overcommit in a calculated opening few rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was competing in only his second appearance at light heavyweight.
By the halfway stage, Parker had kept it close, causing problems for Buatsi, who operated with a tight guard to barely take the better rounds.
The audience's tolerance wore thin because of regular grabbing, with Parker slipping to the canvas several times during grappling.
The underdog finished the stronger, but multiple periods were challenging to assess. Nevertheless, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - judging by this showing - still looks steep.
Hatton Tribute Motivates Manchester Derby Battle
In the secondary fight, Arthur beat Rea by close verdict in a terrific Manchester vs Manchester battle to capture the European light heavyweight championship.
The venue became quiet for a profoundly moving memorial prior to the bout as 10 bells sounded in honor of local hero Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the concluding sound faded, the stillness transitioned to a chant of "there's just one Ricky Hatton".
The fighting inside the ropes equaled the emotion. Arthur was knocked down by a significant blow directly to the face in the initial stage, 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, the opponent - in opposing blue - was marked up soon, noticeable damage forming under his eye as the fighters landed punishing blows.
The challenger, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with powerful upward punches as the intensity seldom decreased over 12 rounds.
The victor barely secured the result with judges scorecards of draw, 115-113 and 115-112, but each competitor were persistent and full of heart, all the qualities their hero the legend embodied.
