British light-heavyweight Joshua Buatsi earned a win yet unimpressive start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a majority decision in a close 10-round fight at the boxing arena.
But Parker found success, connecting with powerful individual punches in a fight where the techniques never quite clicked, with frequent clinching marring the contest.
Two judges had it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my best performance but what do champions do? They prevail," the victor commented in the ringside as he was faced boos from the spectators.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to claim a 20th career victory.
Some ringside observers thought the challenger, thirty-one, deserved the victory but the Derbyshire fighter experienced the second setback of his 28-fight professional journey.
The Champion Claims Victory in Slow Match
His career development has stalled in recent years through promotional disputes and considerable time of inactivity.
Once tipped as the next star of British boxing, he now stands trying to realize his potential - possibly in what feels like a last chance - under new promotional backing.
He began cautiously, with neither fighter ready to be aggressive in a cagey initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second bout at light heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for the favorite, who worked behind a solid defense to edge the better stanzas.
The audience's tolerance wore thin because of frequent holding, with the challenger falling to the floor several times during clinches.
The underdog ended stronger, but several stanzas were challenging to assess. However, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, scoring for Buatsi just one round.
The victor advances, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Memorial Fuels Local Rivalry Fight
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a outstanding local rivalry contest to capture the European light-heavyweight belt.
The stadium grew hushed for a deeply poignant tribute prior to the bout as a bell ceremony tolled in honor of Manchester's own Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised boxing event in the urban center since the passing of Hatton. As the final chime faded, the quiet gave way to a chant of "there's just one Hatton".
The action inside the ropes equaled the emotion. The eventual winner was floored by a left hook directly to the face in the second round, rising shakily to his position on the referee's count.
But the Manchester United-red attire of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was damaged early, heavy swelling developing under his face as the fighters exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the action seldom decreased over twelve rounds.
The victor narrowly won the verdict with official scores of 114-114, narrow lead and 115-112, but both fighters were determined and full of heart, everything their hero the legend embodied.
