Romero weighed in at 186 pounds (84.3kg) initially, and was given two hours to lose the extra pound. On his second trip to the scales, he clocked in at 185.2 pounds (84kg), which saw him relinquish his claim to the belt.
Earlier in the year, in a bout against Luke Rockhold at UFC 221, Romero first weighed in at 188lbs (85.2kg) and was only able to drop to 187.7lbs (85.1kg) before the deadline.
"To miss weight twice, in my opinion, is very unprofessional especially when there is so much on the line for him. He's going to have to assess that," Whittaker said.
The Auckland-born champion said knowing Romero was coming into the bout having missed weight didn't change anything for him, despite the former Olympic wrestling silver medallist losing his chance to win Whittaker's title.
The pair slugged out a five-round epic which saw both land a number of heavy shots, with Whittaker getting the better of the exchanges to win by split decision.
"It didn't bother me at all," Whittaker said of the circumstances coming into the bout. "Especially because I knew this was going to be a hard fight ... it just kind of put him in a corner where he had nothing to lose. I knew he was going to come out hard, the threat and intensity of the fight's all the same."
Whittaker said his body definitely felt like it had been in a fight and, on Friday, he was scheduled to go under the knife to repair a broken bone in his hand - an injury he sustained early in the bout.
"I don't actually know when it happened," Whittaker admitted. "It was just a point in the fight that I remember I couldn't feel my hand from my fingertips to my forearm and I knew that wasn't good."
By the fourth round, it was clear something wasn't right. While he was giving his left-hand lead jab plenty of work, his right hand looked to simply be hanging there - until cries of "just throw it, Robbie" from his corner saw him bring his injured hand back into play.
"I just had to stick to the game plan. Half the game plan was always to initiate a strong jab and pick him apart with that, which he just loved gobbling up. My lead jab was one of my most effective weapons, he just kept walking into it."
Injury aside, Whittaker knew he had done enough to win the fight when the bell rang at the end of the fifth and final round.
Improving his career record to 21-4, undefeated in eight bouts since making the jump from welterweight to middleweight, Whittaker said he intended to give himself time to recover after his surgery, but hoped to fight again this year.
"It is what it is, we'll have to wait and see," he said. "It was a clean break, so the surgery's going to be pretty straightforward on that. We'll just have to play it by ear and see how the rehab goes. Once that's taken care of and I let the UFC know that I'm good, they'll line something up."
