British light heavyweight Joshua Buatsi made a winning yet disappointing start to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a majority decision in a competitive ten-round contest at the Manchester venue.
But Parker found success, connecting with powerful single shots in a contest where the styles didn't complement each other, with constant grabbing marring the action.
The scoring judges judged it 96-94 to the winner, with the third a even score.
"It wasn't my best performance but how do champions respond? They prevail," the victor commented in the ring as he was faced disapproval from the spectators.
The Londoner bounced back after his first professional defeat - an memorable fight with Callum Smith in last winter - to claim a twentieth professional win.
Several boxing experts thought Parker, thirty-one, merited the victory but the British boxer experienced the second defeat of his 28-bout career.
The Champion Earns Victory in Uneventful Affair
His career development has plateaued in the past few years through contract problems and long spells of inactivity.
Once tipped as the next star of British boxing, he now is positioned trying to realize his potential - perhaps in what seems like a last chance - under new promotional backing.
He began cautiously, with both competitors willing to take risks in a cagey initial rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second appearance at light-heavyweight.
By the middle rounds, Parker had kept it close, causing problems for Buatsi, who operated with a protective stance to edge the cleaner rounds.
The spectators' acceptance decreased amid frequent holding, with Parker slipping to the canvas multiple occasions during close exchanges.
Parker ended stronger, but many rounds were hard to judge. However, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi advances, 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 seems difficult.
Ricky Hatton Tribute Inspires Local Rivalry Battle
In the co-main event, Lyndon Arthur beat Rea by close verdict in a excellent Manchester vs Manchester contest to win the European light heavyweight belt.
The stadium fell silent for a extremely emotional tribute before the fight as a bell ceremony rang in memory of the city's native Ricky Hatton, who died 46 years old in September.
It was the first major televised match card in the city since Hatton's passing. As the last bell disappeared, the stillness gave way to a shout of "there's just one Ricky Hatton".
The competition inside the ropes matched the emotion. The eventual winner was sent to the canvas by a significant blow directly to the chin in the second round, standing uncertainly to his footing on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was injured early, heavy swelling forming under his eye as the duo traded punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with effective uppercuts as the action seldom decreased over twelve rounds.
Arthur narrowly won the result with scoring tallies of 114-114, narrow lead and 115-112, but the two athletes were persistent and full of heart, all the qualities their inspiration the legend represented.
