UK light heavyweight Joshua Buatsi made a winning yet disappointing start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a judges' verdict in a tight ten-round contest at the boxing arena.
But the challenger found success, connecting with powerful individual punches in a fight where the techniques failed to mesh, with frequent clinching spoiling the contest.
The scoring judges scored it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my best showing but what defines champions? They win," Buatsi stated in the ring as he was met by negative reactions from the audience.
The British fighter bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Some ringside observers thought Parker, 31, should have been awarded the decision but the contender experienced the second setback of his 28-match career.
The Champion Earns Victory in Uneventful Match
His career development has stalled in recent years through promotional disputes and long spells of lack of fights.
Earlier forecast as the future of the British fight scene, he now stands trying to realize his potential - possibly in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter prepared to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second fight at the higher weight class.
By the middle rounds, Parker had stayed in contention, creating difficulties for Buatsi, who operated with a solid defense to narrowly win the cleaner stanzas.
The crowd's patience wore thin because of constant clinching, with Parker going down various instances during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. Although, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
Buatsi moves on, but with the likes of power puncher Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Hatton Tribute Fuels All-Manchester Fight
In the secondary fight, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding Manchester vs Manchester contest to capture the European light-heavyweight belt.
The arena fell silent for a extremely emotional homage prior to the bout as 10 bells tolled in memory of the city's native Ricky Hatton, who succumbed 46 years old in recently.
It was the first major televised fight night in the urban center since Hatton's passing. As the last bell disappeared, the silence transitioned to a shout of "there's just one Ricky Hatton".
The action during the bout reflected the feeling. The eventual winner was floored by a significant blow flush on the jaw in the initial stage, getting up unsteadily to his position on the referee's count.
But the traditional red attire of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was damaged early, noticeable damage developing under his face as the fighters exchanged powerful shots.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the intensity rarely dropped over twelve rounds.
Arthur edged the decision with scoring tallies of 114-114, 115-113 and clearer margin, but the two athletes were relentless and courageous, exactly what their idol the legend represented.
