UK light heavyweight Buatsi made a winning yet underwhelming debut to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a split decision in a competitive ten-round contest at the boxing arena.
But Parker found success, connecting with powerful clean blows in a contest where the approaches failed to mesh, with excessive holding marring the contest.
The scoring judges had it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best performance but how do champions respond? They prevail," Buatsi said in the ringside as he was met by negative reactions from the audience.
The Londoner got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to claim a 20th career victory.
Several boxing experts thought the challenger, thirty-one, merited the victory but the Derbyshire fighter endured the second defeat of his 28-match professional journey.
The Champion Earns Triumph in Slow Match
Buatsi's progress has plateaued in recent years through contract problems and long spells of lack of fights.
Earlier forecast as the next star of UK boxing, he now is positioned trying to realize his potential - possibly in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors willing to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was participating in his second bout at light heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who operated with a protective stance to narrowly win the more effective periods.
The audience's tolerance wore thin amid constant clinching, with the challenger slipping to the canvas multiple occasions during clinches.
Parker ended stronger, but several stanzas were hard to judge. However, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
The victor moves on, but with the presence of power puncher Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still looks steep.
Hatton Homage Fuels Local Rivalry Contest
In the secondary fight, Arthur overcame Rea by close verdict in a outstanding local rivalry contest to capture the European light-heavyweight title.
The venue grew hushed for a deeply poignant memorial before the fight as 10 bells tolled in memory of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the first major televised fight night in the location since the passing of Hatton. As the concluding sound disappeared, the quiet became a roar of "there's only one Hatton".
The fighting within the ring matched the feeling. The eventual winner was sent to the canvas by a left hook flush on the chin in the early round, getting up unsteadily to his footing on the eight count.
But the red-colored shorts of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was injured quickly, noticeable damage appearing under his orbital area as the pair traded hard punches.
Rea, 27, connected with strong blows but Arthur, 34, fought fire with fire with effective uppercuts as the intensity rarely dropped over 12 rounds.
Arthur barely secured the result with scoring tallies of 114-114, narrow lead and clearer margin, but both fighters were determined and brave, everything their idol the legend stood for.
