British light-heavyweight Buatsi made a winning yet disappointing beginning to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a tight ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, landing some telling clean blows in a match where the approaches failed to mesh, with constant grabbing marring the action.
Two judges had it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my greatest performance but what defines champions? They win," the victor stated in the ringside as he was faced boos from the audience.
The Londoner got back to winning after his first professional defeat - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Several boxing experts felt the challenger, thirty-one, merited the victory but the Derbyshire fighter endured the second defeat of his 28-bout career.
Buatsi Earns Triumph in Uneventful Encounter
The fighter's advancement has stalled in the past few years through contract problems and long spells of inactivity.
Previously predicted as the future of British boxing, he now is positioned trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with neither fighter prepared to overcommit in a cagey early stages.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was participating in his second appearance at the higher weight class.
By the fight's midpoint, Parker had made it competitive, causing problems for Buatsi, who operated with a tight guard to edge the more effective rounds.
The crowd's patience diminished amid regular grabbing, with the challenger falling to the floor several times during close exchanges.
The underdog finished the stronger, but several stanzas were hard to judge. However, former world champion Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, awarding Buatsi just one round.
The victor advances, but with the caliber of power puncher Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Hatton Memorial Inspires Local Rivalry Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a outstanding Manchester vs Manchester battle to capture the continental title.
The stadium grew hushed for a extremely emotional homage before the fight as ten bells sounded in honor of local hero Hatton, who succumbed at age 46 in last month.
It was the premier televised match card in the city since Hatton's death. As the final chime disappeared, the stillness gave way to a chant of "there's only one Ricky Hatton".
The competition during the bout matched the feeling. The eventual winner was floored by a significant blow flush on the jaw in the early round, getting up unsteadily to his footing on the referee's count.
But the Manchester United-red shorts of Arthur soon advanced again. On the opposite side, Rea - in opposing blue - was damaged quickly, significant bruising forming under his eye as the duo exchanged punishing blows.
The challenger, 27, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the pace rarely dropped over the full distance.
Arthur barely secured the decision with scoring tallies of even score, 115-113 and 115-112, but both fighters were persistent and brave, exactly what their hero Hatton stood for.
