The Belgian forward had become something of a forgotten man heading into the 2018-19 campaign.
Origi is, however, now a cult hero on the red half of Merseyside after netting memorable goals in a derby date with Everton and European clashes with Barcelona and Tottenham.
Fresh terms are being mooted for the 24-year-old, whose current contract is due to expire in the summer of 2020.
“[Jurgen] Klopp brought him on against Everton at Anfield with six minutes to go and he scored the winner in the 96th minute. Look what has happened since,” former Reds striker Mellor told the official Liverpool FC magazine.
“He came on to score the winner at Newcastle, netted twice in the 4-0 semi-final comeback against Barcelona and has now scored for Liverpool in a Champions League final victory.
“That is a huge moment for any player so it is credit to him for not giving up on being a Liverpool player. He will have seen, while out on loan, that it’s not great when you leave Liverpool so has proved he is desperate to stay here.
“That has been reflected in the big moments he’s had.”
Mellor added on Origi, who finished the 2018-19 campaign with seven goals to his name: “It says a lot that Liverpool substitutes contributed 17 goals this season, including one in the final.
“It shows there is a squad mentality, that even though players are disappointed to be named on the bench they’ll still come on and perform.
“The attitude those players have shown displays their professionalism and that is important because if substitutes reflect their disappointment and don’t perform, it can have a hugely negative effect on a dressing room.
“Divock Origi is a good example. He must have been thinking he was way down the pecking order and wouldn’t get game time, but he kept working hard in training to impress the manager and coaching staff.”
Origi was signed by Liverpool from Lille in 2014 and is now just three appearances short of reaching 100 for the club.