UK light-heavyweight Buatsi earned a win yet unimpressive start to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a competitive 10-round bout at the boxing arena.
But the challenger found success, landing some telling individual punches in a match where the approaches didn't complement each other, with frequent clinching marring the action.
The officials judged it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my best performance but what do champions do? They win," the victor stated in the ring as he was met by boos from the spectators.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in February - to secure a twentieth professional win.
Several boxing experts thought the challenger, 31, should have been awarded the decision but the contender endured the second loss of his 28-match career.
The Champion Secures Triumph in Dull Affair
His career development has plateaued in recent years through contract problems and extended periods of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now stands trying to realize his potential - perhaps in what seems like a last chance - under fresh management.
He began cautiously, with neither fighter willing to overcommit in a cagey opening few rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in two years ago after suffering a hand injury in the contest, was competing in only his second appearance at light heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for Buatsi, who utilized a solid defense to narrowly win the cleaner rounds.
The crowd's patience decreased because of constant clinching, with the challenger going down various instances during close exchanges.
The underdog closed well, but multiple periods were challenging to assess. Although, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor advances, but with the caliber of hard hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Inspires Local Rivalry Battle
In the co-main event, Arthur beat Bradley Rea by split-decision in a excellent Manchester vs Manchester contest to win the European light-heavyweight championship.
The arena became quiet for a profoundly moving homage ahead of the contest as a bell ceremony sounded in memory of the city's native Ricky Hatton, who succumbed aged 46 in September.
It was the first major televised boxing event in the urban center since Hatton's death. As the last bell faded, the silence gave way to a roar of "there's only one Ricky Hatton".
The competition inside the ropes matched the sentiment. The eventual winner was sent to the canvas by a left hook squarely on the jaw in the early round, rising shakily to his footing on the count of eight.
But the traditional red shorts of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was damaged early, heavy swelling forming under his eye as the fighters exchanged punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with stinging uppercuts as the intensity hardly diminished over the full distance.
The victor barely secured the result with official scores of even score, close margin and 115-112, but the two athletes were relentless and courageous, all the qualities their hero the legend stood for.
