UK light heavyweight Joshua Buatsi earned a win yet disappointing beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a tight ten-round contest at the boxing arena.
But Parker landed significant shots, landing some telling clean blows in a fight where the techniques never quite clicked, with constant grabbing spoiling the fight.
Two judges had it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my best display but how do champions respond? They succeed," Buatsi said in the ring as he was faced disapproval from the spectators.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in last winter - to earn a 20th career victory.
Various analysts believed Parker, 31, deserved the win but the Derbyshire fighter suffered the second setback of his 28-fight career.
The Champion Secures Victory in Uneventful Encounter
Buatsi's progress has slowed in recent years through contract problems and considerable time of inactivity.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to achieve expectations - possibly in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with both competitors prepared to overcommit in a tentative initial rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second fight at the higher weight class.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who operated with a solid defense to barely take the more effective rounds.
The crowd's patience decreased during regular grabbing, with Parker slipping to the canvas several times during grappling.
Parker finished the stronger, but several stanzas were hard to judge. However, ex-champion Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Hatton Tribute Fuels Local Rivalry Contest
In the secondary fight, Lyndon Arthur defeated Rea by close verdict in a excellent all-Manchester contest to secure the continental title.
The arena fell silent for a extremely emotional homage before the fight as ten bells rang in memory of the city's native Hatton, who succumbed 46 years old in September.
It was the first major televised fight night in the city since Hatton's death. As the last bell disappeared, the stillness transitioned to a roar of "only one Ricky Hatton".
The action within the ring matched the sentiment. Arthur was knocked down by a significant blow flush on the chin in the initial stage, rising shakily to his feet on the eight count.
But the red-colored shorts of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was marked up early, noticeable damage appearing under his face as the fighters landed punishing blows.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with powerful upward punches as the pace hardly diminished over the full distance.
Arthur narrowly won the verdict with judges scorecards of 114-114, 115-113 and wider gap, but both fighters were determined and full of heart, all the qualities their idol Hatton embodied.
