UK light-heavyweight Buatsi made a winning yet disappointing beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a close 10-round fight at the Co-op Live Arena.
But Parker found success, landing some telling clean blows in a match where the techniques didn't complement each other, with frequent clinching marring the contest.
The scoring judges judged it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my greatest performance but what defines champions? They succeed," Buatsi stated in the ring as he was faced disapproval from the audience.
The Londoner got back to winning after his first professional defeat - an thrilling contest with his previous opponent in February - to claim a 20th career victory.
Several boxing experts felt Parker, thirty-one, should have been awarded the decision but the British boxer experienced the second loss of his 28-match career.
Buatsi Secures Win in Slow Encounter
His career development has slowed in the past few years through management issues and considerable time of inactivity.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to realize his potential - possibly in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with neither fighter prepared to take risks in a calculated opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after breaking his hand mid-fight, was participating in his second appearance at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for Buatsi, who operated with a tight guard to edge the more effective stanzas.
The crowd's patience decreased amid constant clinching, with Parker going down multiple occasions during close exchanges.
The underdog closed well, but many rounds were hard to judge. Although, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, awarding Buatsi just a single round.
The victor moves on, but with the presence of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Hatton Memorial Inspires Manchester Derby Contest
In the secondary fight, Arthur overcame Bradley Rea by split-decision in a excellent local rivalry contest to secure the continental title.
The stadium became quiet for a extremely emotional tribute ahead of the contest as 10 bells sounded in memory of local hero Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the final chime disappeared, the stillness transitioned to a roar of "there's just one Hatton".
The competition during the bout matched the sentiment. Arthur was floored by a left hook squarely on the face in the second round, rising shakily to his feet on the count of eight.
But the Manchester United-red attire of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was damaged quickly, heavy swelling developing under his eye as the pair exchanged hard punches.
The challenger, 27, delivered effective strikes but Arthur, 34, fought fire with fire with effective uppercuts as the action seldom decreased over the full distance.
The victor edged the result with official scores of draw, 115-113 and 115-112, but both fighters were persistent and full of heart, everything their inspiration the legend stood for.
