British light-heavyweight Buatsi earned a win yet underwhelming debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a tight ten-round contest at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective clean blows in a fight where the techniques never quite clicked, with frequent clinching marring the contest.
The scoring judges judged it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my finest display but how do champions respond? They win," the victor said in the arena as he was greeted with boos from the crowd.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts believed the challenger, 31, should have been awarded the win but the British boxer experienced the second loss of his 28-match professional journey.
Buatsi Earns Win in Slow Encounter
Buatsi's progress has slowed in the past few years through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of British boxing, he now stands trying to achieve expectations - perhaps in what seems like a final roll of the dice - under fresh management.
He started carefully, with the two boxers willing to take risks in a calculated initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after damaging his hand in the contest, was participating in his second bout at the higher weight class.
By the middle rounds, Parker had made it competitive, making life awkward for Buatsi, who operated with a protective stance to barely take the more effective periods.
The spectators' acceptance wore thin because of constant clinching, with the challenger going down various instances during clinches.
The underdog closed well, but many rounds were challenging to assess. Although, former world champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor advances, but with the likes of power puncher David Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Hatton Homage Inspires Manchester Derby Fight
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a outstanding all-Manchester battle to win the European light heavyweight belt.
The arena became quiet for a profoundly moving tribute prior to the bout as ten bells tolled in memory of the city's native Ricky Hatton, who passed away aged 46 in recently.
It was the premier televised boxing event in the urban center since Hatton's death. As the final chime ended, the stillness became a shout of "there's just one Ricky Hatton".
The action inside the ropes matched the sentiment. The eventual winner was sent to the canvas by a left hook flush on the jaw in the initial stage, rising shakily to his position on the count of eight.
But the red-colored equipment of the fighter soon pressed ahead again. Across the ring, Rea - in competitive blue - was damaged soon, noticeable damage forming under his eye as the pair landed powerful shots.
Rea, 27, connected with strong blows but Arthur, 34, fought fire with fire with powerful upward punches as the pace hardly diminished over 12 rounds.
The victor barely secured the decision with scoring tallies of even score, narrow lead and clearer margin, but each competitor were determined and full of heart, exactly what their hero Hatton represented.
