British light heavyweight Buatsi made a winning yet unimpressive beginning to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a majority decision in a close 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a fight where the techniques failed to mesh, with constant grabbing spoiling the fight.
Two judges judged it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest showing but what defines champions? They prevail," Buatsi stated in the ringside as he was greeted with negative reactions from the audience.
The Londoner returned to winning ways after his first professional defeat - an epic battle with Callum Smith in February - to earn a twentieth professional win.
Various analysts felt Parker, 31, merited the victory but the contender endured the second setback of his 28-bout career.
The Champion Earns Win in Slow Match
Buatsi's progress has plateaued in recent years through promotional disputes and long spells of lack of fights.
Once tipped as the next star of British boxing, he now stands trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers prepared to take risks in a calculated opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in two years ago after damaging his hand mid-fight, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, making life awkward for the favorite, who operated with a tight guard to narrowly win the cleaner periods.
The spectators' acceptance wore thin amid constant clinching, with the challenger going down several times during clinches.
Parker ended stronger, but many rounds were difficult to score. Nevertheless, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the likes of power puncher Benavidez, and Russian champion Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Homage Fuels All-Manchester Contest
In the secondary fight, Arthur overcame Rea by split-decision in a outstanding local rivalry battle to win the European light-heavyweight belt.
The arena fell silent for a extremely emotional homage ahead of the contest as ten bells sounded in honor of local hero Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast match card in the city since Hatton's death. As the final chime faded, the silence gave way to a shout of "there's just one Hatton".
The competition within the ring equaled the sentiment. The eventual winner was floored by a significant blow flush on the jaw in the early round, rising shakily to his position on the count of eight.
But the red-colored shorts of Arthur soon advanced again. From the other corner, Rea - in opposing blue - was injured quickly, noticeable damage forming under his eye as the fighters exchanged hard punches.
The challenger, 27, delivered effective strikes but Arthur, 34, matched the intensity with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur barely secured the result with judges scorecards of even score, 115-113 and wider gap, but each competitor were persistent and brave, all the qualities their hero the legend represented.
