UK light-heavyweight Buatsi secured a victory yet underwhelming debut to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a judges' verdict in a tight 10-round contest at the boxing arena.
But Parker found success, delivering effective individual punches in a fight where the styles failed to mesh, with excessive holding marring the fight.
The scoring judges scored it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest performance but what do champions do? They win," Buatsi stated in the ringside as he was met by boos from the audience.
The Londoner got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to secure a 20th pro success.
Some ringside observers felt the challenger, thirty-one, should have been awarded the victory but the British boxer experienced the second defeat of his 28-fight professional journey.
The Champion Secures Win in Slow Affair
His career development has slowed in recent years through promotional disputes and considerable time of lack of fights.
Once tipped as the future of UK boxing, he now finds himself trying to achieve expectations - maybe in what seems like a ultimate opportunity - under recent support.
He started carefully, with the two boxers ready to be aggressive in a tentative initial rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in two years ago after breaking his hand during the bout, was participating in his second appearance at the higher weight class.
By the fight's midpoint, Parker had kept it close, making life awkward for the favorite, who operated with a protective stance to edge the more effective rounds.
The spectators' acceptance decreased during constant clinching, with Parker falling to the floor several times during clinches.
The underdog finished the stronger, but several stanzas were hard to judge. Nevertheless, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
The victor advances, but with the likes of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Hatton Homage Inspires Manchester Derby Battle
In the secondary fight, Arthur beat Bradley Rea by divided decision in a excellent all-Manchester fight to secure the European light heavyweight belt.
The stadium fell silent for a deeply poignant memorial before the fight as ten bells sounded in memory of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised fight night in the urban center since the passing of Hatton. As the concluding sound ended, the stillness gave way to a roar of "there's only one Ricky Hatton".
The competition within the ring reflected the sentiment. Arthur was sent to the canvas by a significant blow flush on the chin in the early round, standing uncertainly to his feet on the referee's count.
But the Manchester United-red shorts of the fighter soon came forward again. Across the ring, Rea - in competitive blue - was marked up early, significant bruising developing under his eye as the duo exchanged powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, responded in kind with stinging uppercuts as the pace rarely dropped over the full distance.
Arthur narrowly won the verdict with judges scorecards of 114-114, narrow lead and wider gap, but both fighters were relentless and courageous, everything their idol the legend embodied.
