UK light heavyweight Buatsi secured a victory yet unimpressive start to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a majority decision in a close 10-round contest at the boxing arena.
But the challenger had his moments, landing some telling individual punches in a match where the approaches never quite clicked, with constant grabbing disrupting the contest.
The scoring judges scored it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my best display but what do champions do? They prevail," the victor commented in the arena as he was greeted with negative reactions from the spectators.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a twentieth professional win.
Some ringside observers felt the challenger, 31, deserved the victory but the contender endured the second setback of his 28-bout career.
The Champion Earns Win in Dull Match
His career development has stalled in the past few years through management issues and long spells of lack of fights.
Earlier forecast as the next star of UK boxing, he now is positioned trying to realize his potential - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter willing to be aggressive in a cagey early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after damaging his hand mid-fight, was participating in his second fight at the higher weight class.
By the fight's midpoint, Parker had kept it close, causing problems for Buatsi, who operated with a tight guard to barely take the better stanzas.
The crowd's patience decreased because of frequent holding, with the challenger going down multiple occasions during close exchanges.
Parker ended stronger, but multiple periods were hard to judge. Although, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi moves on, but with the caliber of hard hitter Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - based on this performance - still appears challenging.
Hatton Tribute Inspires All-Manchester Contest
In the secondary fight, Arthur defeated Rea by close verdict in a terrific all-Manchester fight to win the European light heavyweight title.
The venue grew hushed for a extremely emotional tribute ahead of the contest as ten bells sounded in remembrance of local hero Ricky Hatton, who passed away aged 46 in recently.
It was the premier televised boxing event in the urban center since Hatton's death. As the last bell ended, the quiet became a chant of "only one Hatton".
The action within the ring equaled the emotion. Arthur was sent to the canvas by a left hook flush on the jaw in the early round, standing uncertainly to his footing on the eight count.
But the red-colored equipment of the fighter soon came forward again. On the opposite side, Rea - in rival City blue - was damaged soon, noticeable damage appearing under his face as the fighters traded powerful shots.
Rea, 27, landed power punches but the champion, 34, matched the intensity with effective uppercuts as the pace hardly diminished over the full distance.
Arthur barely secured the result with judges scorecards of draw, 115-113 and 115-112, but both fighters were persistent and full of heart, exactly what their hero the legend stood for.
