British light-heavyweight Buatsi made a winning yet disappointing debut to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a tight ten-round bout at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling individual punches in a fight where the approaches never quite clicked, with excessive holding disrupting the fight.
The scoring judges had it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my best showing but what do champions do? They succeed," Buatsi said in the ringside as he was greeted with boos from the audience.
The British fighter got back to winning after his maiden career setback - an thrilling contest with his previous opponent in last winter - to earn a 20th pro success.
Various analysts felt the challenger, 31, deserved the victory but the contender endured the second loss of his 28-bout professional journey.
The Champion Claims Victory in Uneventful Encounter
The fighter's advancement has stalled in recent times through contract problems and long spells of lack of fights.
Once tipped as the next star of British boxing, he now stands trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under fresh management.
He began cautiously, with both competitors willing to overcommit in a calculated early stages.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second appearance at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to narrowly win the more effective rounds.
The spectators' acceptance wore thin during regular grabbing, with Parker going down various instances during clinches.
The underdog closed well, but many rounds were hard to judge. Although, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor advances, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Hatton Tribute Inspires Local Rivalry Fight
In the secondary fight, Arthur defeated Rea by split-decision in a excellent all-Manchester fight to secure the European light heavyweight belt.
The arena became quiet for a profoundly moving tribute before the fight as a bell ceremony rang in memory of the city's native Ricky Hatton, who passed away 46 years old in recently.
It was the first major televised fight night in the location since Hatton's death. As the final chime ended, the stillness transitioned to a shout of "only one Hatton".
The action inside the ropes matched the feeling. The eventual winner was floored by a left hook squarely on the face in the initial stage, standing uncertainly to his footing on the eight count.
But the traditional red shorts of the fighter soon advanced again. From the other corner, Rea - in opposing blue - was marked up quickly, noticeable damage developing under his face as the fighters traded punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with stinging uppercuts as the pace hardly diminished over 12 rounds.
Arthur narrowly won the verdict with judges scorecards of draw, narrow lead and clearer margin, but each competitor were relentless and full of heart, everything their inspiration the legend represented.
