UK light-heavyweight Buatsi earned a win yet underwhelming beginning to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a tight ten-round contest at the Co-op Live Arena.
But Parker had his moments, landing some telling single shots in a fight where the styles didn't complement each other, with excessive holding spoiling the contest.
The scoring judges had it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my best showing but how do champions respond? They prevail," the victor commented in the arena as he was faced boos from the crowd.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in February - to secure a 20th pro success.
Some ringside observers thought Parker, 31, merited the decision but the Derbyshire fighter experienced the second loss of his 28-match career.
The Champion Earns Victory in Uneventful Affair
Buatsi's progress has stalled in recent times through contract problems and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now stands trying to realize his potential - possibly in what feels like a last chance - under fresh management.
He started carefully, with both competitors willing to take risks in a cagey early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after damaging his hand during the bout, was competing in only his second bout at light heavyweight.
By the middle rounds, the underdog had kept it close, making life awkward for the favorite, who operated with a protective stance to barely take the cleaner periods.
The audience's tolerance decreased because of regular grabbing, with the challenger going down various instances during close exchanges.
Parker finished the stronger, but many rounds were difficult to score. Although, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi advances, but with the likes of hard hitter Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Homage Fuels Local Rivalry Battle
In the secondary fight, Arthur overcame Bradley Rea by close verdict in a excellent all-Manchester battle to secure the continental title.
The stadium became quiet for a extremely emotional homage before the fight as ten bells tolled in honor of Manchester's own Ricky Hatton, who died at age 46 in last month.
It was the first major televised match card in the urban center since Hatton's death. As the concluding sound disappeared, the stillness became a shout of "there's only one Hatton".
The action within the ring reflected the feeling. The eventual winner was knocked down by a left hook flush on the chin in the second round, standing uncertainly to his footing on the referee's count.
But the traditional red attire of Arthur soon advanced again. On the opposite side, the opponent - in opposing blue - was damaged soon, heavy swelling appearing under his eye as the fighters exchanged punishing blows.
The challenger, 27, connected with strong blows but the champion, 34, matched the intensity with stinging uppercuts as the intensity hardly diminished over 12 rounds.
Arthur edged the verdict with scoring tallies of draw, narrow lead and wider gap, but each competitor were persistent and brave, all the qualities their inspiration the legend stood for.
