UK light-heavyweight Buatsi earned a win yet disappointing beginning to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a judges' verdict in a competitive 10-round fight at the boxing arena.
But Parker had his moments, delivering effective clean blows in a contest where the techniques never quite clicked, with excessive holding spoiling the action.
The scoring judges had it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my finest display but how do champions respond? They succeed," Buatsi commented in the arena as he was met by disapproval from the spectators.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Several boxing experts believed the challenger, 31, merited the victory but the contender endured the second setback of his 28-bout career.
Buatsi Claims Triumph in Slow Encounter
The fighter's advancement has stalled in the past few years through promotional disputes and considerable time of lack of fights.
Once tipped as the future of UK boxing, he now is positioned trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under recent support.
He started carefully, with neither fighter willing to be aggressive in a calculated initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after breaking his hand mid-fight, was making just his second fight at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for Buatsi, who operated with a protective stance to barely take the cleaner rounds.
The spectators' acceptance diminished during regular grabbing, with the challenger falling to the floor multiple occasions during close exchanges.
Parker finished the stronger, but several stanzas were hard to judge. However, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
Buatsi moves on, but with the likes of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Homage Inspires All-Manchester Battle
In the co-main event, Arthur beat Rea by close verdict in a terrific local rivalry contest to capture the European light heavyweight championship.
The arena became quiet for a deeply poignant memorial prior to the bout as a bell ceremony tolled in memory of local hero Ricky Hatton, who passed away 46 years old in last month.
It was the premier televised boxing event in the location since Hatton's passing. As the concluding sound disappeared, the quiet became a chant of "there's only one Ricky Hatton".
The competition inside the ropes equaled the emotion. Arthur was sent to the canvas by a significant blow squarely on the chin in the second round, standing uncertainly to his feet on the eight count.
But the traditional red equipment of Arthur soon came forward again. Across the ring, Rea - in competitive blue - was damaged early, heavy swelling forming under his orbital area as the pair traded punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, 34, responded in kind with powerful upward punches as the action hardly diminished over the full distance.
Arthur edged the verdict with official scores of even score, 115-113 and clearer margin, but the two athletes were determined and courageous, exactly what their inspiration the legend represented.
