UK light-heavyweight Buatsi earned a win yet underwhelming debut to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a split decision in a competitive ten-round fight at the boxing arena.
But Parker found success, delivering effective individual punches in a match where the techniques didn't complement each other, with frequent clinching spoiling the contest.
Two judges judged it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my best display but what do champions do? They succeed," the victor commented in the ring as he was greeted with boos from the audience.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to secure a twentieth professional win.
Various analysts thought the challenger, thirty-one, merited the victory but the British boxer suffered the second setback of his 28-fight professional journey.
Buatsi Claims Victory in Slow Encounter
The fighter's advancement has stalled in the past few years through contract problems and considerable time of inactivity.
Previously predicted as the next star of the British fight scene, he now stands trying to achieve expectations - perhaps in what appears to be a last chance - under fresh management.
He opened tentatively, with the two boxers ready to overcommit in a calculated initial rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after breaking his hand in the contest, was participating in his second fight at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, creating difficulties for Buatsi, who worked behind a protective stance to barely take the more effective periods.
The crowd's patience wore thin during frequent holding, with Parker going down several times during close exchanges.
The underdog ended stronger, but many rounds were challenging to assess. However, former world champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, scoring for Buatsi just a single round.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - judging by this showing - still looks steep.
Hatton Homage Inspires Manchester Derby Fight
In the secondary fight, Arthur beat Rea by divided decision in a terrific all-Manchester battle to win the European light heavyweight belt.
The venue fell silent for a extremely emotional memorial ahead of the contest as 10 bells rang in honor of local hero Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast match card in the location since Hatton's passing. As the final chime disappeared, the quiet gave way to a chant of "only one Hatton".
The competition inside the ropes matched the feeling. Arthur was sent to the canvas by a left hook flush on the chin in the second round, getting up unsteadily to his feet on the eight count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. Across the ring, the opponent - in rival City blue - was injured quickly, significant bruising developing under his eye as the duo traded hard punches.
Rea, 27, connected with strong blows but Arthur, 34, fought fire with fire with stinging uppercuts as the pace rarely dropped over the full distance.
Arthur edged the verdict with scoring tallies of 114-114, 115-113 and clearer margin, but both fighters were determined and courageous, exactly what their idol the legend represented.
