UK light-heavyweight Buatsi made a winning yet disappointing debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a majority decision in a competitive 10-round bout at the boxing arena.
But Parker had his moments, connecting with powerful individual punches in a contest where the techniques failed to mesh, with excessive holding spoiling the contest.
Two judges judged it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my finest showing but how do champions respond? They win," the victor said in the ringside as he was faced boos from the crowd.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Some ringside observers believed the challenger, 31, merited the win but the contender endured the second setback of his 28-fight professional journey.
Buatsi Claims Win in Slow Encounter
The fighter's advancement has stalled in recent years through promotional disputes and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now finds himself trying to achieve expectations - perhaps in what feels like a final roll of the dice - under fresh management.
He opened tentatively, with both competitors willing to be aggressive in a calculated initial rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after breaking his hand during the bout, was making just his second bout at light heavyweight.
By the middle rounds, Parker had kept it close, causing problems for Buatsi, who worked behind a tight guard to barely take the better stanzas.
The crowd's patience diminished during regular grabbing, with the challenger falling to the floor several times during clinches.
The underdog ended stronger, but many rounds were hard to judge. However, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
The victor advances, but with the presence of power puncher Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Homage Motivates Manchester Derby Contest
In the secondary fight, Arthur beat Rea by split-decision in a excellent local rivalry battle to win the continental belt.
The stadium became quiet for a extremely emotional homage before the fight as ten bells sounded in remembrance of local hero Hatton, who died at age 46 in last month.
It was the first major televised fight night in the location since Hatton's death. As the concluding sound disappeared, the silence became a shout of "there's just one Hatton".
The action during the bout reflected the sentiment. Arthur was sent to the canvas by a powerful punch flush on the chin in the second round, rising shakily to his footing on the eight count.
But the red-colored equipment of Arthur soon advanced again. Across the ring, the opponent - in opposing blue - was damaged soon, noticeable damage developing under his face as the fighters traded hard punches.
Rea, 27, delivered effective strikes but Arthur, 34, fought fire with fire with effective uppercuts as the action seldom decreased over twelve rounds.
The victor narrowly won the verdict with official scores of even score, close margin and clearer margin, but both fighters were relentless and brave, all the qualities their inspiration the legend stood for.
