UK light heavyweight Buatsi earned a win yet underwhelming start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a competitive 10-round fight at the Manchester venue.
But the challenger had his moments, connecting with powerful single shots in a fight where the techniques didn't complement each other, with frequent clinching marring the contest.
The scoring judges had it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my finest display but what do champions do? They win," Buatsi said in the arena as he was faced negative reactions from the crowd.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in February - to earn a twentieth professional win.
Various analysts felt Parker, 31, merited the win but the British boxer experienced the second loss of his 28-fight career.
The Champion Earns Win in Dull Affair
His career development has plateaued in recent years through management issues and considerable time of lack of fights.
Once tipped as the next star of UK boxing, he now stands trying to achieve expectations - maybe in what appears to be a last chance - under recent support.
He began cautiously, with neither fighter prepared to overcommit in a cagey opening few rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in two years ago after suffering a hand injury during the bout, was making just his second bout at light-heavyweight.
By the middle rounds, the underdog had kept it close, causing problems for Buatsi, who operated with a tight guard to barely take the cleaner stanzas.
The crowd's patience decreased amid regular grabbing, with the challenger slipping to the canvas various instances during close exchanges.
Parker ended stronger, but several stanzas were challenging to assess. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi advances, but with the presence of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Hatton Tribute Motivates Local Rivalry Fight
In the supporting bout, Arthur overcame Rea by divided decision in a excellent Manchester vs Manchester fight to secure the continental championship.
The stadium became quiet for a extremely emotional tribute before the fight as a bell ceremony sounded in remembrance of Manchester's own Hatton, who died 46 years old in recently.
It was the premier televised boxing event in the urban center since Hatton's death. As the last bell disappeared, the quiet gave way to a roar of "there's just one Hatton".
The fighting during the bout equaled the emotion. Arthur was knocked down by a significant blow directly to the face in the initial stage, rising shakily to his position on the referee's count.
But the Manchester United-red shorts of the fighter soon came forward again. Across the ring, the opponent - in competitive blue - was damaged soon, noticeable damage appearing under his eye as the fighters landed hard punches.
Rea, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the action rarely dropped over 12 rounds.
The victor barely secured the decision with scoring tallies of draw, 115-113 and 115-112, but each competitor were relentless and full of heart, all the qualities their idol the legend stood for.
