UK light heavyweight Joshua Buatsi earned a win yet underwhelming start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a competitive 10-round fight at the Manchester venue.
But Parker had his moments, delivering effective clean blows in a fight where the techniques never quite clicked, with constant grabbing spoiling the fight.
Two judges scored it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my greatest performance but what defines champions? They win," Buatsi said in the ringside as he was faced disapproval from the spectators.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to earn a 20th career victory.
Several boxing experts believed Parker, 31, should have been awarded the victory but the British boxer experienced the second setback of his 28-match career.
The Champion Earns Victory in Dull Affair
The fighter's advancement has plateaued in recent times through management issues and long spells of inactivity.
Once tipped as the rising talent of UK boxing, he now finds himself trying to realize his potential - perhaps in what feels like a last chance - under new promotional backing.
He started carefully, with the two boxers prepared to be aggressive in a calculated initial rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after damaging his hand mid-fight, was competing in only his second bout at the higher weight class.
By the middle rounds, the underdog had kept it close, making life awkward for the favorite, who utilized a protective stance to edge the better periods.
The audience's tolerance decreased during regular grabbing, with Parker falling to the floor several times during clinches.
The underdog closed well, but several stanzas were hard to judge. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the likes of power puncher Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still appears challenging.
Hatton Tribute Motivates Manchester Derby Battle
In the co-main event, Lyndon Arthur beat Rea by split-decision in a excellent Manchester vs Manchester contest to capture the continental title.
The stadium fell silent for a extremely emotional memorial prior to the bout as 10 bells rang in remembrance of the city's native Ricky Hatton, who succumbed at age 46 in September.
It was the premier televised boxing event in the location since Hatton's death. As the final chime disappeared, the silence gave way to a roar of "only one Hatton".
The fighting during the bout reflected the feeling. The eventual winner was sent to the canvas by a powerful punch flush on the chin in the early round, getting up unsteadily to his position on the count of eight.
But the traditional red shorts of the fighter soon advanced again. From the other corner, Rea - in opposing blue - was injured early, noticeable damage forming under his orbital area as the fighters exchanged hard punches.
The challenger, 27, landed power punches but Arthur, 34, responded in kind with powerful upward punches as the pace rarely dropped over the full distance.
Arthur edged the decision with official scores of even score, close margin and 115-112, but both fighters were persistent and brave, all the qualities their inspiration the legend embodied.
