British light heavyweight Buatsi earned a win yet disappointing debut to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a tight 10-round fight at the boxing arena.
But the challenger found success, landing some telling single shots in a match where the styles never quite clicked, with constant grabbing disrupting the fight.
The officials scored it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my best display but what defines champions? They succeed," Buatsi said in the arena as he was faced negative reactions from the spectators.
The Londoner bounced back after his initial pro loss - an thrilling contest with his previous opponent in last winter - to earn a 20th pro success.
Various analysts believed Parker, 31, should have been awarded the decision but the British boxer suffered the second setback of his 28-bout career.
Buatsi Claims Triumph in Uneventful Encounter
The fighter's advancement has plateaued in recent times through promotional disputes and extended periods of lack of fights.
Once tipped as the next star of British boxing, he now stands trying to achieve expectations - perhaps in what seems like a last chance - under fresh management.
He began cautiously, with both competitors willing to take risks in a cagey early stages.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in two years ago after damaging his hand in the contest, was making just his second bout at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for the favorite, who utilized a protective stance to narrowly win the cleaner periods.
The crowd's patience diminished during regular grabbing, with Parker slipping to the canvas several times during close exchanges.
Parker finished the stronger, but many rounds were difficult to score. However, former world champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the caliber of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the co-main event, Arthur overcame Rea by divided decision in a outstanding all-Manchester fight to capture the European light heavyweight title.
The stadium grew hushed for a profoundly moving tribute ahead of the contest as 10 bells rang in memory of local hero Ricky Hatton, who died 46 years old in last month.
It was the premier televised fight night in the urban center since the passing of Hatton. As the last bell disappeared, the silence gave way to a shout of "there's just one Hatton".
The competition during the bout reflected the sentiment. The eventual winner was sent to the canvas by a left hook squarely on the jaw in the initial stage, rising shakily to his feet on the count of eight.
But the traditional red shorts of the fighter soon advanced again. From the other corner, the opponent - in opposing blue - was injured early, significant bruising developing under his orbital area as the fighters exchanged powerful shots.
Rea, 27, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the action seldom decreased over the full distance.
The victor edged the result with official scores of even score, narrow lead and clearer margin, but both fighters were relentless and full of heart, everything their hero the legend represented.
