UK light heavyweight Buatsi earned a win yet unimpressive debut to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a competitive ten-round fight at the boxing arena.
But Parker found success, landing some telling individual punches in a match where the styles failed to mesh, with constant grabbing spoiling the action.
Two judges judged it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my finest performance but what do champions do? They win," the victor said in the ringside as he was faced boos from the crowd.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts thought Parker, thirty-one, should have been awarded the victory but the contender endured the second setback of his 28-bout professional journey.
The Champion Claims Win in Dull Match
Buatsi's progress has slowed in recent years through promotional disputes and extended periods of lack of fights.
Earlier forecast as the future of British boxing, he now stands trying to realize his potential - perhaps in what appears to be a last chance - under fresh management.
He started carefully, with neither fighter prepared to take risks in a cagey opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was making just his second appearance at the higher weight class.
By the middle rounds, Parker had stayed in contention, causing problems for Buatsi, who worked behind a protective stance to barely take the better rounds.
The crowd's patience diminished during frequent holding, with the challenger going down multiple occasions during close exchanges.
The underdog ended stronger, but many rounds were hard to judge. Although, former world champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi advances, but with the likes of heavy hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still seems difficult.
Hatton Tribute Fuels Manchester Derby Battle
In the supporting bout, Arthur defeated Bradley Rea by split-decision in a terrific Manchester vs Manchester battle to secure the European light-heavyweight belt.
The arena became quiet for a deeply poignant tribute ahead of the contest as 10 bells rang in remembrance of local hero Hatton, who passed away 46 years old in September.
It was the first major televised fight night in the city since Hatton's death. As the concluding sound ended, the stillness became a roar of "only one Hatton".
The competition inside the ropes matched the feeling. The eventual winner was knocked down by a significant blow flush on the jaw in the second round, rising shakily to his position on the referee's count.
But the Manchester United-red shorts of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was marked up soon, significant bruising forming under his eye as the pair exchanged powerful shots.
Rea, 27, landed power punches but Arthur, 34, matched the intensity with stinging uppercuts as the pace hardly diminished over twelve rounds.
Arthur narrowly won the decision with scoring tallies of draw, 115-113 and clearer margin, but each competitor were determined and full of heart, all the qualities their idol Hatton embodied.
