British light heavyweight Buatsi earned a win yet underwhelming debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a competitive 10-round bout at the boxing arena.
But Parker had his moments, delivering effective single shots in a match where the techniques didn't complement each other, with frequent clinching marring the contest.
The scoring judges scored it 96-94 for to the winner, with the third a even score.
"It wasn't my best showing but how do champions respond? They succeed," Buatsi commented in the arena as he was met by boos from the crowd.
The British fighter bounced back after his initial pro loss - an epic battle with his previous opponent in last winter - to claim a twentieth professional win.
Several boxing experts felt the challenger, thirty-one, should have been awarded the victory but the contender experienced the second defeat of his 28-match professional journey.
The Champion Earns Victory in Dull Match
His career development has plateaued in recent years through contract problems and long spells of lack of fights.
Earlier forecast as the rising talent of British boxing, he now stands trying to achieve expectations - possibly in what seems like a ultimate opportunity - under fresh management.
He started carefully, with both competitors ready to overcommit in a calculated initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after breaking his hand in the contest, was making just his second appearance at the higher weight class.
By the halfway stage, Parker had kept it close, making life awkward for Buatsi, who worked behind a protective stance to narrowly win the cleaner periods.
The spectators' acceptance wore thin because of constant clinching, with the challenger falling to the floor several times during clinches.
Parker closed well, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the caliber of power puncher Benavidez, and Russian champion Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Hatton Tribute Fuels All-Manchester Contest
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a excellent Manchester vs Manchester contest to secure the European light-heavyweight belt.
The venue fell silent for a deeply poignant memorial before the fight as ten bells tolled in memory of local hero Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised match card in the location since Hatton's passing. As the concluding sound ended, the silence gave way to a shout of "there's only one Ricky Hatton".
The fighting during the bout reflected the emotion. The eventual winner was knocked down by a powerful punch directly to the chin in the initial stage, rising shakily to his footing on the referee's count.
But the Manchester United-red attire of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was damaged soon, significant bruising appearing under his face as the fighters traded hard punches.
The challenger, 27, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity hardly diminished over the full distance.
The victor barely secured the result with official scores of 114-114, 115-113 and wider gap, but the two athletes were determined and courageous, exactly what their inspiration the legend stood for.
