British light-heavyweight Buatsi made a winning yet disappointing start to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a close 10-round contest at the boxing arena.
But the challenger had his moments, landing some telling individual punches in a match where the styles failed to mesh, with excessive holding marring the action.
The scoring judges had it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They win," Buatsi said in the arena as he was faced disapproval from the crowd.
The Londoner got back to winning after his initial pro loss - an thrilling contest with his previous opponent in last winter - to earn a 20th career victory.
Various analysts believed Parker, thirty-one, deserved the win but the Derbyshire fighter suffered the second defeat of his 28-fight career.
Buatsi Secures Triumph in Dull Affair
Buatsi's progress has slowed in recent years through management issues and extended periods of lack of fights.
Once tipped as the rising talent of UK boxing, he now finds himself trying to realize his potential - perhaps in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers willing to be aggressive in a tentative initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after breaking his hand during the bout, was making just his second appearance at light heavyweight.
By the halfway stage, the underdog had kept it close, making life awkward for the favorite, who operated with a protective stance to narrowly win the cleaner rounds.
The crowd's patience decreased because of regular grabbing, with Parker going down various instances during clinches.
Parker finished the stronger, but several stanzas were difficult to score. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, giving Buatsi just only one stanza.
Buatsi advances, but with the likes of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Tribute Fuels All-Manchester Fight
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester battle to win the European light-heavyweight belt.
The stadium became quiet for a profoundly moving tribute before the fight as a bell ceremony tolled in memory of the city's native Hatton, who died at age 46 in September.
It was the premier televised fight night in the urban center since Hatton's passing. As the concluding sound ended, the silence transitioned to a roar of "there's only one Hatton".
The action during the bout matched the sentiment. The eventual winner was floored by a significant blow squarely on the chin in the early round, rising shakily to his position on the referee's count.
But the Manchester United-red attire of the fighter soon pressed ahead again. Across the ring, the opponent - in rival City blue - was damaged quickly, heavy swelling developing under his eye as the fighters exchanged hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with powerful upward punches as the intensity hardly diminished over 12 rounds.
The victor narrowly won the decision with judges scorecards of 114-114, narrow lead and 115-112, but the two athletes were determined and full of heart, all the qualities their idol the legend stood for.
