UK light-heavyweight Buatsi made a winning yet underwhelming beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a competitive ten-round contest at the Co-op Live Arena.
But the challenger had his moments, delivering effective clean blows in a contest where the styles never quite clicked, with constant grabbing disrupting the contest.
Two judges had it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest showing but what do champions do? They prevail," the victor said in the ring as he was faced negative reactions from the spectators.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in last winter - to claim a 20th career victory.
Various analysts believed the challenger, 31, deserved the decision but the Derbyshire fighter suffered the second defeat of his 28-fight career.
Buatsi Secures Win in Slow Encounter
The fighter's advancement has stalled in recent times through contract problems and long spells of inactivity.
Earlier forecast as the next star of British boxing, he now stands trying to realize his potential - maybe in what seems like a final roll of the dice - under recent support.
He opened tentatively, with neither fighter ready to take risks in a tentative opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second appearance at light heavyweight.
By the halfway stage, the underdog had stayed in contention, causing problems for Buatsi, who operated with a tight guard to edge the more effective rounds.
The spectators' acceptance wore thin during frequent holding, with the challenger falling to the floor multiple occasions during clinches.
Parker ended stronger, but many rounds were challenging to assess. Nevertheless, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Tribute Motivates All-Manchester Contest
In the co-main event, Arthur overcame Rea by close verdict in a terrific Manchester vs Manchester contest to win the European light heavyweight belt.
The stadium became quiet for a extremely emotional tribute before the fight as 10 bells sounded in honor of Manchester's own Ricky Hatton, who passed away 46 years old in recently.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the concluding sound disappeared, the stillness gave way to a shout of "there's only one Ricky Hatton".
The competition inside the ropes equaled the emotion. Arthur was floored by a left hook flush on the face in the early round, standing uncertainly to his feet on the count of eight.
But the red-colored attire of the fighter soon came forward again. On the opposite side, the opponent - in competitive blue - was damaged early, heavy swelling developing under his eye as the fighters exchanged powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur narrowly won the verdict with scoring tallies of even score, close margin and clearer margin, but the two athletes were relentless and full of heart, exactly what their inspiration the legend stood for.
