British light-heavyweight Buatsi secured a victory yet underwhelming start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a competitive 10-round fight at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective individual punches in a match where the styles didn't complement each other, with excessive holding marring the action.
The scoring judges had it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my greatest performance but what defines champions? They prevail," Buatsi stated in the ring as he was greeted with boos from the audience.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Several boxing experts felt Parker, thirty-one, merited the decision but the British boxer suffered the second setback of his 28-match career.
Buatsi Secures Triumph in Slow Match
His career development has plateaued in recent times through management issues and long spells of inactivity.
Earlier forecast as the rising talent of British boxing, he now stands trying to realize his potential - perhaps in what seems like a ultimate opportunity - under fresh management.
He opened tentatively, with neither fighter willing to overcommit in a calculated opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in two years ago after breaking his hand during the bout, was participating in his second appearance at light heavyweight.
By the fight's midpoint, the underdog had kept it close, creating difficulties for Buatsi, who operated with a protective stance to barely take the better stanzas.
The crowd's patience diminished during constant clinching, with Parker falling to the floor various instances during grappling.
Parker closed well, but many rounds were hard to judge. Although, former world champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the likes of hard hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Hatton Tribute Inspires Local Rivalry Fight
In the co-main event, Arthur beat Rea by divided decision in a outstanding local rivalry battle to capture the European light heavyweight belt.
The stadium fell silent for a extremely emotional memorial prior to the bout as a bell ceremony sounded in memory of Manchester's own Ricky Hatton, who succumbed aged 46 in recently.
It was the first major televised boxing event in the city since Hatton's passing. As the last bell disappeared, the silence transitioned to a roar of "only one Ricky Hatton".
The fighting during the bout reflected the feeling. The eventual winner was floored by a significant blow flush on the jaw in the initial stage, getting up unsteadily to his feet on the referee's count.
But the traditional red attire of Arthur soon advanced again. Across the ring, the opponent - in rival City blue - was injured early, noticeable damage appearing under his orbital area as the pair exchanged powerful shots.
Rea, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the action hardly diminished over 12 rounds.
Arthur barely secured the verdict with scoring tallies of even score, 115-113 and clearer margin, but both fighters were determined and brave, exactly what their idol Hatton embodied.
