British light heavyweight Buatsi made a winning yet underwhelming beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a tight 10-round bout at the Co-op Live Arena.
But Parker found success, delivering effective clean blows in a match where the approaches never quite clicked, with constant grabbing disrupting the action.
The officials judged it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my finest performance but how do champions respond? They prevail," the victor commented in the arena as he was greeted with boos from the audience.
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 believed Parker, 31, deserved the win but the Derbyshire fighter experienced the second setback of his 28-bout career.
Buatsi Earns Win in Dull Match
Buatsi's progress has slowed in recent years through contract problems and considerable time of inactivity.
Earlier forecast as the next star of British boxing, he now finds himself trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under fresh management.
He opened tentatively, with both competitors prepared to overcommit in a cagey early stages.
Parker, whose single earlier setback came at super middleweight against John Ryder in two years ago after breaking his hand in the contest, was participating in his second bout at the higher weight class.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who operated with a solid defense to barely take the more effective stanzas.
The spectators' acceptance decreased amid frequent holding, with Parker falling to the floor various instances during clinches.
Parker ended stronger, but many rounds were difficult to score. Although, former world champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
Buatsi moves on, but with the presence of power puncher Benavidez, and Russian star Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Hatton Tribute Motivates Manchester Derby Contest
In the secondary fight, Lyndon Arthur overcame Bradley Rea by split-decision in a excellent local rivalry fight to secure the continental title.
The venue became quiet for a profoundly moving tribute prior to the bout as 10 bells rang in memory of the city's native Ricky Hatton, who died aged 46 in recently.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the final chime ended, the silence became a chant of "there's just one Ricky Hatton".
The competition during the bout reflected the sentiment. Arthur was floored by a left hook directly to the chin in the initial stage, standing uncertainly to his footing on the count of eight.
But the red-colored shorts of the fighter soon advanced again. On the opposite side, the opponent - in competitive blue - was damaged soon, heavy swelling developing under his eye as the pair landed hard punches.
Rea, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with stinging uppercuts as the action seldom decreased over twelve rounds.
The victor edged the verdict with scoring tallies of draw, narrow lead and clearer margin, but the two athletes were relentless and brave, everything their hero the legend stood for.
