British light-heavyweight Buatsi made a winning yet underwhelming beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a split decision in a tight ten-round bout at the boxing arena.
But the challenger landed significant shots, landing some telling individual punches in a fight where the styles never quite clicked, with frequent clinching disrupting the fight.
The officials judged it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my finest showing but what defines champions? They win," the victor said in the arena as he was greeted with boos from the audience.
The British fighter got back to winning after his maiden career setback - an epic battle with his previous opponent in last winter - to earn a twentieth professional win.
Some ringside observers felt the challenger, 31, should have been awarded the decision but the contender endured the second defeat of his 28-bout professional journey.
The Champion Claims Win in Slow Affair
Buatsi's progress has stalled in the past few years through management issues and considerable time of lack of fights.
Once tipped as the next star of British boxing, he now stands trying to realize his potential - maybe in what seems like a last chance - under fresh management.
He started carefully, with the two boxers ready to overcommit in a cagey initial rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was making just his second appearance at the higher weight class.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who operated with a protective stance to narrowly win the cleaner periods.
The audience's tolerance diminished because of regular grabbing, with Parker falling to the floor several times during grappling.
Parker finished the stronger, but several stanzas were difficult to score. Nevertheless, former world champion Carl Frampton - working with Dazn - had it strongly leaning for Parker, awarding Buatsi just one round.
Buatsi moves on, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Hatton Tribute Motivates Local Rivalry Contest
In the secondary fight, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific local rivalry fight to secure the European light-heavyweight belt.
The stadium grew hushed for a profoundly moving tribute prior to the bout as 10 bells tolled in remembrance of the city's native Ricky Hatton, who died aged 46 in recently.
It was the premier televised boxing event in the city since Hatton's death. As the concluding sound faded, the stillness gave way to a roar of "there's only one Ricky Hatton".
The fighting inside the ropes reflected the feeling. Arthur was sent to the canvas by a left hook directly to the chin in the second round, getting up unsteadily to his footing on the eight count.
But the traditional red attire of Arthur soon advanced again. From the other corner, Rea - in rival City blue - was damaged early, significant bruising developing under his eye as the pair traded powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, fought fire with fire with powerful upward punches as the intensity hardly diminished over the full distance.
The victor narrowly won the decision with judges scorecards of 114-114, 115-113 and wider gap, but the two athletes were determined and courageous, all the qualities their hero Hatton embodied.
