British light-heavyweight Buatsi secured a victory yet disappointing start to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a majority decision in a close 10-round fight at the boxing arena.
But Parker found success, connecting with powerful individual punches in a contest where the approaches didn't complement each other, with excessive holding disrupting the contest.
The officials had it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my best showing but what do champions do? They succeed," the victor said in the arena as he was met by negative reactions from the audience.
The Londoner returned to winning ways after his initial pro loss - an epic battle with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts thought the challenger, thirty-one, deserved the decision but the British boxer endured the second defeat of his 28-match career.
The Champion Claims Victory in Uneventful Affair
The fighter's advancement has stalled in recent times through management issues and considerable time of inactivity.
Earlier forecast as the next star of the British fight scene, he now stands trying to achieve expectations - possibly in what seems like a last chance - under new promotional backing.
He started carefully, with both competitors prepared to take risks in a tentative initial rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in two years ago after breaking his hand during the bout, was participating in his second fight at light heavyweight.
By the halfway stage, Parker had made it competitive, making life awkward for the favorite, who operated with a protective stance to barely take the cleaner rounds.
The audience's tolerance wore thin during constant clinching, with Parker falling to the floor several times during close exchanges.
The underdog finished the stronger, but many rounds were difficult to score. Although, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, scoring for Buatsi just a single round.
Buatsi advances, but with the caliber of heavy hitter Benavidez, and Russian champion Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Hatton Memorial Inspires Manchester Derby Fight
In the supporting bout, Arthur defeated Rea by close verdict in a terrific Manchester vs Manchester contest to win the continental belt.
The stadium became quiet for a extremely emotional homage prior to the bout as ten bells rang in remembrance of local hero Ricky Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the concluding sound faded, the quiet transitioned to a chant of "only one Hatton".
The competition within the ring reflected the feeling. Arthur was sent to the canvas by a left hook flush on the jaw in the second round, standing uncertainly to his position on the count of eight.
But the traditional red attire of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was marked up early, noticeable damage developing under his orbital area as the pair exchanged punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the pace rarely dropped over twelve rounds.
Arthur barely secured the decision with judges scorecards of draw, narrow lead and 115-112, but each competitor were relentless and courageous, everything their inspiration the legend embodied.
