UK light-heavyweight Buatsi made a winning yet underwhelming debut to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a split decision in a close ten-round contest at the Manchester venue.
But Parker found success, connecting with powerful single shots in a contest where the styles failed to mesh, with frequent clinching marring the fight.
The scoring judges judged it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my best performance but what do champions do? They succeed," Buatsi commented in the ringside as he was met by negative reactions from the audience.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a twentieth professional win.
Some ringside observers felt the challenger, 31, deserved the win but the British boxer suffered the second setback of his 28-bout professional journey.
The Champion Claims Win in Slow Match
The fighter's advancement has plateaued in the past few years through contract problems and extended periods of lack of fights.
Previously predicted as the future of the British fight scene, he now finds himself trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under new promotional backing.
He started carefully, with neither fighter ready to take risks in a calculated early stages.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in two years ago after suffering a hand injury in the contest, was competing in only his second fight at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who utilized a solid defense to edge the cleaner rounds.
The crowd's patience diminished because of regular grabbing, with Parker falling to the floor multiple occasions during grappling.
Parker ended stronger, but multiple periods were hard to judge. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
The victor moves on, but with the likes of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - on this evidence - still seems difficult.
Hatton Homage Inspires Manchester Derby Fight
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a excellent all-Manchester fight to win the European light-heavyweight belt.
The stadium fell silent for a deeply poignant tribute before the fight as ten bells sounded in memory of Manchester's own Ricky Hatton, who died at age 46 in last month.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the final chime faded, the silence transitioned to a roar of "only one Hatton".
The competition within the ring matched the sentiment. Arthur was floored by a left hook flush on the face in the initial stage, rising shakily to his feet on the count of eight.
But the red-colored equipment of Arthur soon pressed ahead again. Across the ring, Rea - in competitive blue - was marked up soon, heavy swelling developing under his face as the pair exchanged powerful shots.
Rea, 27, landed power punches but Arthur, thirty-four, responded in kind with effective uppercuts as the pace hardly diminished over twelve rounds.
Arthur barely secured the verdict with scoring tallies of even score, narrow lead and clearer margin, but each competitor were relentless and courageous, all the qualities their idol the legend represented.
