UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a split decision in a close ten-round fight at the Manchester venue.
But Parker found success, landing some telling single shots in a fight where the techniques failed to mesh, with constant grabbing marring the contest.
The officials judged it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my best display but what do champions do? They prevail," the victor said in the ring as he was faced boos from the audience.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to earn a twentieth professional win.
Some ringside observers felt the challenger, 31, merited the win but the British boxer suffered the second loss of his 28-fight professional journey.
The Champion Claims Triumph in Slow Match
Buatsi's progress has plateaued in the past few years through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of the British fight scene, he now stands trying to realize his potential - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter willing to be aggressive in a calculated initial rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in two years ago after damaging his hand during the bout, was making just his second appearance at the higher weight class.
By the halfway stage, the underdog had made it competitive, causing problems for the favorite, who worked behind a tight guard to edge the more effective rounds.
The crowd's patience diminished because of frequent holding, with the challenger falling to the floor various instances during grappling.
The underdog finished the stronger, but many rounds were difficult to score. However, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi progresses, but with the likes of power puncher Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - based on this performance - still appears challenging.
Hatton Memorial Inspires All-Manchester Battle
In the co-main event, Arthur beat Bradley Rea by split-decision in a excellent all-Manchester fight to secure the European light-heavyweight title.
The stadium grew hushed for a extremely emotional homage prior to the bout as a bell ceremony sounded in honor of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the first major televised boxing event in the urban center since Hatton's passing. As the final chime faded, the silence gave way to a chant of "there's just one Hatton".
The fighting within the ring equaled the feeling. The eventual winner was floored by a significant blow flush on the chin in the early round, getting up unsteadily to his position on the count of eight.
But the Manchester United-red shorts of Arthur soon advanced again. On the opposite side, the opponent - in rival City blue - was damaged quickly, significant bruising forming under his orbital area as the duo landed hard punches.
The challenger, 27, delivered effective strikes but the champion, thirty-four, fought fire with fire with stinging uppercuts as the pace seldom decreased over twelve rounds.
The victor barely secured the decision with judges scorecards of even score, close margin and wider gap, but each competitor were persistent and courageous, everything their hero Hatton represented.
