British light heavyweight Buatsi earned a win yet unimpressive beginning to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a tight ten-round fight at the Co-op Live Arena.
But Parker landed significant shots, delivering effective single shots in a fight where the techniques didn't complement each other, with constant grabbing disrupting the fight.
The officials judged it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my greatest display but what defines champions? They prevail," Buatsi stated in the ringside as he was greeted with boos from the crowd.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in last winter - to claim a 20th career victory.
Various analysts felt the challenger, 31, deserved the win but the contender suffered the second loss of his 28-match career.
The Champion Claims Win in Dull Match
His career development has stalled in recent times through contract problems and considerable time of lack of fights.
Earlier forecast as the future of British boxing, he now stands trying to realize his potential - perhaps in what appears to be a last chance - under fresh management.
He started carefully, with both competitors willing to overcommit in a calculated early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was participating in his second bout at light heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for Buatsi, who operated with a solid defense to narrowly win the cleaner periods.
The audience's tolerance decreased amid frequent holding, with the challenger going down various instances during clinches.
Parker finished the stronger, but multiple periods were difficult to score. Although, former world champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi progresses, but with the presence of power puncher Benavidez, and Russian star Bivol ruling the division, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Tribute Inspires All-Manchester Fight
In the secondary fight, Arthur defeated Bradley Rea by split-decision in a terrific all-Manchester battle to win the European light-heavyweight belt.
The arena fell silent for a deeply poignant tribute ahead of the contest as ten bells sounded in honor of Manchester's own Ricky Hatton, who died aged 46 in last month.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the concluding sound ended, the silence gave way to a roar of "there's only one Hatton".
The competition within the ring reflected the sentiment. Arthur was sent to the canvas by a significant blow squarely on the chin in the early round, rising shakily to his position on the eight count.
But the Manchester United-red equipment of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was injured early, significant bruising developing under his orbital area as the pair traded punishing blows.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with effective uppercuts as the action seldom decreased over twelve rounds.
Arthur narrowly won the result with scoring tallies of 114-114, narrow lead and clearer margin, but both fighters were relentless and full of heart, exactly what their idol the legend stood for.
