The Spaniard felt aggrieved after referee Chris Foy awarded the hosts a spot-kick in the 32nd minute when former Baggies man Shane Long appeared to trail his leg in a coming together with Craig Dawson.
Ben Foster saved Nikica Jelavic's resulting effort, only for Liam Rosenior to head home the rebound to put Hull in front.
Soon afterwards, Long doubled the advantage when the visitors failed to clear a corner, while Mel felt that his team were hard done by again when James Morrison went down under a challenge from Jake Livermore in the area 20 minutes from time.
This time, though, Foy waved away the vociferous appeals, leaving the West Brom boss less than impressed.
"We played well for the first 25 minutes but the penalty changed the game," he told reporters.
"I don't know if it was or wasn't a penalty, but I have one question, why we did not get a penalty from a similar incident involving James Morrison in the second half?
"I think it was a bad decision from the referee.
"In the second half, it's the same situation (as the Long incident) in the other box and the referee doesn't whistle.
"Shane Long is a good professional, a good striker. I think he only wants the best for his team, it's normal."
Hull boss Steve Bruce, however, moved to defend Long, insisting that his striker would never dive.
"It might be a soft penalty but Chris Foy is less than 10 yards away when he gives the decision so maybe he's seen contact," Bruce said.
"Longy's never going to dive, he's as genuine as they come but when you move as quickly as he does then the slightest little tap you're going to go over because you're travelling at such pace all the time.
"He was instrumental in getting us the penalty and obviously he's got a terrific goal.
"It's always a bit surreal playing against your old club, it's a strange sensation, but I'm sure when he's sitting down tonight he'll enjoy the day."
The only negative for Bruce was Long's withdrawal 12 minutes from time after he suffered a knock, but he is hopeful that the injury will not prove to be too much of a problem.
"We'll see how he is but we don't think it's too serious," Bruce added.
"We think it's a kick to his knee but for him to come off there must be something wrong."