UK light heavyweight Buatsi made a winning yet disappointing debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a tight 10-round fight at the Manchester venue.
But Parker found success, connecting with powerful individual punches in a match where the styles failed to mesh, with excessive holding marring the contest.
The scoring judges scored it 96-94 to the winner, with the third a 95-95.
"It wasn't my finest performance but what defines champions? They prevail," the victor stated in the ring as he was greeted with negative reactions from the spectators.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in February - to secure a twentieth professional win.
Various analysts felt Parker, thirty-one, should have been awarded the victory but the British boxer experienced the second loss of his 28-match professional journey.
Buatsi Earns Win in Slow Affair
Buatsi's progress has plateaued in the past few years through contract problems and extended periods of lack of fights.
Previously predicted as the future of British boxing, he now finds himself trying to realize his potential - possibly in what feels like a ultimate opportunity - under recent support.
He started carefully, with neither fighter ready to take risks in a cagey early stages.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in two years ago after breaking his hand mid-fight, was making just his second bout at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, making life awkward for Buatsi, who utilized a tight guard to narrowly win the better periods.
The crowd's patience diminished amid constant clinching, with the challenger slipping to the canvas multiple occasions during close exchanges.
The underdog finished the stronger, but many rounds were hard to judge. Although, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the presence of power puncher Benavidez, and Russian star Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Hatton Tribute Motivates All-Manchester Fight
In the co-main event, Arthur overcame Bradley Rea by split-decision in a excellent Manchester vs Manchester battle to capture the European light heavyweight belt.
The stadium became quiet for a deeply poignant tribute prior to the bout as 10 bells tolled in remembrance of Manchester's own Hatton, who died 46 years old in September.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the final chime ended, the quiet became a chant of "there's just one Hatton".
The fighting during the bout matched the feeling. The eventual winner was sent to the canvas by a left hook squarely on the face in the initial stage, standing uncertainly to his position on the referee's count.
But the traditional red attire of the fighter soon advanced again. On the opposite side, the opponent - in rival City blue - was marked up quickly, significant bruising forming under his eye as the duo traded powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur narrowly won the result with judges scorecards of draw, 115-113 and 115-112, but the two athletes were determined and courageous, everything their hero the legend stood for.
