UK light-heavyweight Joshua Buatsi secured a victory yet disappointing start to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a competitive ten-round contest at the Manchester venue.
But the challenger landed significant shots, delivering effective single shots in a fight where the techniques never quite clicked, with constant grabbing marring the fight.
The scoring judges scored it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my greatest showing but what do champions do? They prevail," the victor stated in the ring as he was greeted with boos from the spectators.
The British fighter bounced back after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a 20th pro success.
Various analysts felt the challenger, thirty-one, merited the victory but the British boxer endured the second loss of his 28-match professional journey.
The Champion Secures Triumph in Slow Affair
His career development has stalled in recent times through contract problems and long spells of lack of fights.
Earlier forecast as the future of the British fight scene, he now is positioned trying to realize his potential - possibly in what feels like a last chance - under new promotional backing.
He began cautiously, with neither fighter ready to be aggressive in a cagey early stages.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second bout at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for the favorite, who utilized a tight guard to edge the better stanzas.
The crowd's patience wore thin amid constant clinching, with the challenger falling to the floor several times during clinches.
Parker closed well, but many rounds were difficult to score. However, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for Parker, giving Buatsi just one round.
The victor advances, but with the likes of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - based on this performance - still seems difficult.
Hatton Homage Fuels Local Rivalry Fight
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a excellent all-Manchester fight to capture the European light-heavyweight belt.
The venue grew hushed for a profoundly moving tribute prior to the bout as ten bells sounded in memory of local hero Hatton, who succumbed aged 46 in September.
It was the first major televised match card in the city since Hatton's death. As the last bell disappeared, the stillness became a shout of "only one Ricky Hatton".
The fighting inside the ropes matched the sentiment. The eventual winner was floored by a significant blow flush on the face in the second round, rising shakily to his feet on the count of eight.
But the traditional red equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in opposing blue - was marked up early, noticeable damage appearing under his eye as the duo landed hard punches.
The challenger, twenty-seven, landed power punches but the champion, 34, responded in kind with effective uppercuts as the action hardly diminished over the full distance.
Arthur edged the decision with official scores of 114-114, 115-113 and 115-112, but the two athletes were determined and courageous, exactly what their inspiration the legend embodied.
