UK light-heavyweight Joshua Buatsi earned a win yet underwhelming debut to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a tight 10-round fight at the Manchester venue.
But the challenger had his moments, landing some telling individual punches in a fight where the approaches failed to mesh, with frequent clinching marring the fight.
The scoring judges scored it 96-94 to the winner, with the third a 95-95.
"It wasn't my finest showing but what defines champions? They prevail," Buatsi stated in the arena as he was met by boos from the crowd.
The British fighter returned to winning ways after his maiden career setback - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Several boxing experts felt Parker, 31, merited the win but the contender suffered the second setback of his 28-bout professional journey.
The Champion Secures Victory in Slow Match
His career development has stalled in recent years through contract problems and extended periods of inactivity.
Once tipped as the next star of the British fight scene, he now stands trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under recent support.
He opened tentatively, with neither fighter willing to be aggressive in a calculated opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in two years ago after breaking his hand during the bout, was competing in only his second bout at light-heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who operated with a tight guard to edge the better periods.
The crowd's patience diminished during regular grabbing, with Parker falling to the floor various instances during grappling.
The underdog finished the stronger, but multiple periods were hard to judge. Although, former world champion Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor advances, but with the likes of hard hitter Benavidez, and Russian champion Bivol ruling the division, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Homage Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a excellent local rivalry battle to win the European light-heavyweight championship.
The arena became quiet for a deeply poignant homage ahead of the contest as a bell ceremony rang in memory of the city's native Hatton, who passed away 46 years old in September.
It was the premier televised boxing event in the location since the passing of Hatton. As the final chime ended, the silence became a shout of "there's just one Ricky Hatton".
The action during the bout equaled the emotion. The eventual winner was floored by a powerful punch squarely on the jaw in the early round, standing uncertainly to his footing on the eight count.
But the traditional red equipment of Arthur soon pressed ahead again. From the other corner, Rea - in competitive blue - was marked up early, noticeable damage appearing under his eye as the pair landed hard punches.
The challenger, 27, connected with strong blows but Arthur, 34, matched the intensity with stinging uppercuts as the pace hardly diminished over 12 rounds.
Arthur edged the verdict with scoring tallies of draw, narrow lead and 115-112, but the two athletes were persistent and full of heart, all the qualities their hero Hatton stood for.
