UK light heavyweight Joshua Buatsi earned a win yet disappointing beginning to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a judges' verdict in a competitive 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful individual punches in a contest where the styles never quite clicked, with constant grabbing spoiling the contest.
The scoring judges had it 96-94 for to the winner, with the remaining official a 95-95.
"It wasn't my best showing but what do champions do? They succeed," the victor said in the ring as he was met by boos from the crowd.
The Londoner bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to secure a 20th pro success.
Several boxing experts thought the challenger, 31, merited the victory but the British boxer endured the second loss of his 28-match career.
The Champion Claims Victory in Uneventful Match
Buatsi's progress has slowed in the past few years through promotional disputes and long spells of inactivity.
Earlier forecast as the next star of British boxing, he now finds himself trying to realize his potential - possibly in what feels like a final roll of the dice - under recent support.
He started carefully, with both competitors prepared to overcommit in a cagey initial rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in two years ago after suffering a hand injury during the bout, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had stayed in contention, making life awkward for Buatsi, who utilized a tight guard to edge the more effective periods.
The audience's tolerance decreased during regular grabbing, with the challenger going down multiple occasions during grappling.
Parker finished the stronger, but many rounds were hard to judge. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi advances, but with the presence of hard hitter Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Memorial Fuels Local Rivalry Contest
In the co-main event, Arthur overcame Rea by split-decision in a excellent all-Manchester fight to capture the European light heavyweight title.
The venue became quiet for a extremely emotional tribute prior to the bout as ten bells sounded in honor of local hero Ricky Hatton, who passed away aged 46 in September.
It was the first major televised fight night in the location since Hatton's passing. As the concluding sound ended, the silence gave way to a roar of "there's just one Ricky Hatton".
The fighting during the bout equaled the sentiment. The eventual winner was floored by a significant blow directly to the face in the initial stage, rising shakily to his position on the referee's count.
But the Manchester United-red equipment of the fighter soon came forward again. On the opposite side, the opponent - in opposing blue - was injured early, noticeable damage developing under his face as the fighters landed hard punches.
Rea, twenty-seven, landed power punches but Arthur, 34, responded in kind with effective uppercuts as the action hardly diminished over the full distance.
The victor narrowly won the result with official scores of draw, 115-113 and wider gap, but both fighters were persistent and full of heart, exactly what their inspiration the legend embodied.
