UK light-heavyweight Buatsi made a winning yet underwhelming start to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a tight 10-round fight at the Manchester venue.
But Parker found success, delivering effective single shots in a fight where the techniques failed to mesh, with frequent clinching disrupting the fight.
Two judges scored it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest performance but what defines champions? They win," the victor said in the arena as he was met by boos from the crowd.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers thought the challenger, thirty-one, deserved the win but the British boxer suffered the second defeat of his 28-fight professional journey.
The Champion Earns Victory in Dull Affair
Buatsi's progress has plateaued in recent times through contract problems and long spells of inactivity.
Previously predicted as the future of the British fight scene, he now is positioned trying to achieve expectations - possibly in what feels like a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers willing to be aggressive in a calculated opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in two years ago after breaking his hand in the contest, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, Parker had made it competitive, creating difficulties for the favorite, who worked behind a tight guard to barely take the cleaner stanzas.
The spectators' acceptance wore thin during regular grabbing, with the challenger falling to the floor several times during clinches.
The underdog finished the stronger, but several stanzas were hard to judge. Although, previous titleholder Carl Frampton - working with Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the likes of heavy hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Hatton Memorial Motivates Manchester Derby Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by close verdict in a terrific local rivalry contest to secure the European light heavyweight title.
The stadium grew hushed for a deeply poignant homage ahead of the contest as ten bells tolled in remembrance of local hero Ricky Hatton, who succumbed at age 46 in September.
It was the first major televised match card in the urban center since Hatton's death. As the concluding sound disappeared, the stillness gave way to a roar of "there's only one Ricky Hatton".
The action inside the ropes matched the emotion. The eventual winner was knocked down by a left hook squarely on the chin in the second round, getting up unsteadily to his footing on the eight count.
But the traditional red attire of the fighter soon came forward again. From the other corner, the opponent - in opposing blue - was injured soon, noticeable damage developing under his eye as the duo exchanged punishing blows.
The challenger, 27, landed power punches but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the pace seldom decreased over the full distance.
Arthur narrowly won the result with scoring tallies of even score, narrow lead and 115-112, but each competitor were determined and brave, exactly what their hero the legend represented.
