UK light heavyweight Joshua Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a close ten-round contest at the boxing arena.
But the challenger found success, connecting with powerful individual punches in a match where the techniques never quite clicked, with excessive holding marring the fight.
The scoring judges judged it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my best display but what defines champions? They win," Buatsi commented in the arena as he was greeted with boos from the crowd.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Some ringside observers felt the challenger, thirty-one, merited the win but the contender suffered the second loss of his 28-bout professional journey.
Buatsi Earns Triumph in Uneventful Match
Buatsi's progress has slowed in the past few years through promotional disputes and considerable time of lack of fights.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to realize his potential - maybe in what appears to be a final roll of the dice - under recent support.
He started carefully, with both competitors willing to take risks in a calculated opening few rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in two years ago after breaking his hand during the bout, was making just his second fight at light-heavyweight.
By the halfway stage, the underdog had kept it close, creating difficulties for the favorite, who operated with a tight guard to barely take the more effective stanzas.
The spectators' acceptance wore thin amid constant clinching, with the challenger falling to the floor several times during grappling.
The underdog finished the stronger, but many rounds were challenging to assess. However, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, giving Buatsi just one round.
The victor advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Memorial Fuels All-Manchester Battle
In the co-main event, Arthur defeated Rea by split-decision in a excellent local rivalry fight to capture the European light-heavyweight belt.
The arena grew hushed for a profoundly moving memorial before the fight as 10 bells tolled in memory of the city's native Hatton, who died at age 46 in last month.
It was the premier televised boxing event in the urban center since Hatton's passing. As the concluding sound ended, the quiet transitioned to a chant of "there's just one Hatton".
The action within the ring reflected the feeling. Arthur was knocked down by a significant blow squarely on the jaw in the initial stage, getting up unsteadily to his position on the eight count.
But the red-colored shorts of the fighter soon came forward again. On the opposite side, Rea - in rival City blue - was damaged early, noticeable damage developing under his orbital area as the duo exchanged powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with effective uppercuts as the intensity hardly diminished over the full distance.
Arthur edged the result with scoring tallies of draw, close margin and 115-112, but the two athletes were persistent and courageous, all the qualities their inspiration the legend embodied.
