British light-heavyweight Buatsi secured a victory yet disappointing beginning to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a majority decision in a tight ten-round contest at the boxing arena.
But the challenger landed significant shots, delivering effective single shots in a contest where the styles didn't complement each other, with frequent clinching spoiling the contest.
Two judges had it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my finest display but what do champions do? They succeed," the victor said in the ring as he was greeted with disapproval from the spectators.
The British fighter got back to winning after his first professional defeat - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Several boxing experts felt Parker, 31, deserved the victory but the contender experienced the second setback of his 28-match professional journey.
The Champion Secures Triumph in Dull Match
Buatsi's progress has slowed in recent times through contract problems and extended periods of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to achieve expectations - possibly in what seems like a final roll of the dice - under fresh management.
He began cautiously, with the two boxers willing to overcommit in a cagey opening few rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after suffering a hand injury in the contest, was competing in only his second appearance at the higher weight class.
By the middle rounds, the underdog had stayed in contention, causing problems for the favorite, who worked behind a protective stance to narrowly win the cleaner rounds.
The audience's tolerance wore thin during frequent holding, with Parker falling to the floor several times during clinches.
Parker finished the stronger, but multiple periods were difficult to score. However, ex-champion Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, awarding Buatsi just one round.
Buatsi moves on, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still looks steep.
Hatton Memorial Inspires Manchester Derby Battle
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to capture the continental championship.
The arena became quiet for a extremely emotional memorial prior to the bout as ten bells rang in remembrance of local hero Ricky Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the concluding sound disappeared, the silence gave way to a roar of "only one Hatton".
The fighting during the bout reflected the sentiment. Arthur was knocked down by a significant blow flush on the chin in the early round, rising shakily to his footing on the count of eight.
But the Manchester United-red equipment of the fighter soon pressed ahead again. Across the ring, the opponent - in opposing blue - was marked up early, noticeable damage developing under his face as the pair landed powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with effective uppercuts as the pace rarely dropped over the full distance.
Arthur narrowly won the decision with scoring tallies of even score, close margin and clearer margin, but the two athletes were relentless and full of heart, everything their hero the legend embodied.
