British light heavyweight Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a competitive 10-round fight at the Co-op Live Arena.
But Parker found success, delivering effective clean blows in a contest where the styles failed to mesh, with frequent clinching disrupting the fight.
The officials judged it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my best display but what defines champions? They prevail," Buatsi said in the ring as he was met by negative reactions from the crowd.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Several boxing experts felt the challenger, 31, merited the victory but the British boxer endured the second defeat of his 28-bout professional journey.
The Champion Earns Victory in Uneventful Match
The fighter's advancement has stalled in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to achieve expectations - possibly in what seems like a last chance - under new promotional backing.
He began cautiously, with the two boxers prepared to be aggressive in a tentative early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was participating in his second bout at the higher weight class.
By the middle rounds, the underdog had stayed in contention, causing problems for Buatsi, who utilized a solid defense to barely take the more effective periods.
The crowd's patience wore thin during frequent holding, with the challenger falling to the floor various instances during close exchanges.
The underdog finished the stronger, but several stanzas were hard to judge. However, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just a single round.
The victor moves on, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - judging by this showing - still looks steep.
Hatton Memorial Inspires All-Manchester Battle
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a terrific local rivalry contest to capture the European light heavyweight belt.
The arena became quiet for a extremely emotional memorial prior to the bout as a bell ceremony sounded in memory of the city's native Hatton, who passed away at age 46 in recently.
It was the first major televised fight night in the urban center since Hatton's death. As the concluding sound ended, the quiet gave way to a shout of "there's only one Hatton".
The fighting inside the ropes matched the feeling. The eventual winner was knocked down by a powerful punch squarely on the jaw in the second round, rising shakily to his position on the eight count.
But the traditional red shorts of the fighter soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured soon, noticeable damage developing under his orbital area as the pair landed hard punches.
Rea, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the action seldom decreased over 12 rounds.
The victor narrowly won the verdict with judges scorecards of 114-114, close margin and 115-112, but the two athletes were determined and courageous, all the qualities their idol Hatton stood for.
