UK light heavyweight Joshua Buatsi earned a win yet underwhelming beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a tight 10-round fight at the Manchester venue.
But the challenger found success, connecting with powerful single shots in a fight where the styles never quite clicked, with excessive holding spoiling the contest.
Two judges judged it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my best performance but what defines champions? They win," Buatsi said in the ringside as he was met by boos from the spectators.
The Londoner bounced back after his maiden career setback - an thrilling contest with his previous opponent in last winter - to claim a 20th pro success.
Various analysts believed Parker, 31, should have been awarded the victory but the British boxer experienced the second setback of his 28-fight career.
The Champion Earns Win in Slow Match
His career development has slowed in recent times through contract problems and long spells of inactivity.
Earlier forecast as the future of the British fight scene, he now stands trying to achieve expectations - maybe in what feels like a last chance - under fresh management.
He opened tentatively, with the two boxers ready to overcommit in a tentative initial rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second appearance at the higher weight class.
By the middle rounds, the underdog had kept it close, causing problems for Buatsi, who operated with a solid defense to barely take the more effective stanzas.
The crowd's patience decreased during frequent holding, with the challenger falling to the floor several times during close exchanges.
The underdog finished the stronger, but several stanzas were hard to judge. Although, previous titleholder Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, giving Buatsi just a single round.
Buatsi advances, but with the presence of power puncher David Benavidez, and Russian star Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Hatton Tribute Fuels Local Rivalry Fight
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a outstanding local rivalry fight to secure the European light-heavyweight belt.
The venue became quiet for a profoundly moving tribute prior to the bout as ten bells rang in memory of local hero Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast match card in the city since Hatton's passing. As the final chime ended, the silence gave way to a roar of "only one Hatton".
The competition during the bout equaled the sentiment. Arthur was floored by a left hook flush on the chin in the initial stage, getting up unsteadily to his position on the eight count.
But the traditional red attire of the fighter soon came forward again. On the opposite side, the opponent - in rival City blue - was injured quickly, heavy swelling forming under his orbital area as the fighters landed powerful shots.
The challenger, 27, delivered effective strikes but Arthur, 34, fought fire with fire with stinging uppercuts as the pace hardly diminished over the full distance.
The victor edged the decision with judges scorecards of even score, 115-113 and clearer margin, but each competitor were determined and full of heart, exactly what their inspiration Hatton represented.
