UK light heavyweight Buatsi earned a win yet unimpressive start to life under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a competitive ten-round contest at the Manchester venue.
But the challenger found success, delivering effective clean blows in a contest where the techniques didn't complement each other, with frequent clinching disrupting the contest.
Two judges scored it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my greatest performance but what defines champions? They prevail," the victor said in the ring as he was faced boos from the crowd.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to earn a twentieth professional win.
Various analysts believed Parker, thirty-one, deserved the decision but the Derbyshire fighter endured the second defeat of his 28-match career.
The Champion Earns Win in Dull Encounter
His career development has stalled in the past few years through contract problems and considerable time of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to achieve expectations - perhaps in what seems like a final roll of the dice - under fresh management.
He started carefully, with both competitors willing to be aggressive in a cagey opening few rounds.
The challenger, whose sole prior loss came at super middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was competing in only his second appearance at the higher weight class.
By the halfway stage, Parker had stayed in contention, causing problems for Buatsi, who worked behind a protective stance to barely take the more effective periods.
The audience's tolerance decreased because of constant clinching, with the challenger going down several times during clinches.
The underdog closed well, but many rounds were challenging to assess. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just one round.
Buatsi moves on, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Hatton Tribute Fuels Local Rivalry Battle
In the secondary fight, Arthur defeated Bradley Rea by split-decision in a terrific Manchester vs Manchester contest to capture the European light-heavyweight title.
The arena grew hushed for a profoundly moving memorial before the fight as ten bells tolled in honor of Manchester's own Ricky Hatton, who died 46 years old in last month.
It was the premier televised fight night in the urban center since Hatton's death. As the final chime ended, the quiet became a shout of "there's just one Ricky Hatton".
The competition inside the ropes reflected the feeling. Arthur was sent to the canvas by a left hook squarely on the face in the second round, standing uncertainly to his position on the count of eight.
But the red-colored equipment of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was injured soon, noticeable damage appearing under his orbital area as the fighters exchanged powerful shots.
The challenger, twenty-seven, landed power punches but the champion, 34, responded in kind with stinging uppercuts as the intensity hardly diminished over twelve rounds.
The victor edged the decision with scoring tallies of even score, close margin and clearer margin, but both fighters were relentless and courageous, exactly what their hero Hatton embodied.
