One of the tweets says: "KG called me a cancer patient, I'm pissed because, u know how many people died from cancer, and he's tossing it like it's a joke."
Garnett is known for his trash-talking on the court. Villanueva suffers from alopecia universalis, a medical condition that results in hair loss. The Detroit forward does not have hair on his head.
The Pistons confirmed that Villanueva posted the tweets but had no further comment.
Garnett called the incident "a major miscommunication."
Before the Celtics' win over the Bucks on Wednesday he released a statement.
"My comment to Charlie Villanueva was in fact 'You are cancerous to your team and our league,'" Garnett said. "I would never be insensitive to the brave struggle that cancer patients endure. I have lost loved ones to this deadly disease and have a family member currently undergoing treatment. I would never say anything that distasteful. The game of life is far bigger than the game of basketball."
After the game Garnett said: "I'm a passionate player. If you don't like the way I am ... I play this way the last 15, 16 years. I leave it on the court."
Villanueva had posted multiple tweets saying that Garnett's alleged comments hit home.
"I wouldn't even trip about that, but a cancer patient, I know way 2 many people who passed away from it, and I have a special place 4 those," the tweet reads.
Garnett scored 22 points and had six rebounds in the Celtics' 109-86 victory. Villanueva had 17 points and seven rebounds off the bench for the winless Pistons. Villanueva was ready to take the battle off the court.
"KG talks alot of crap, he's prob never been in a fight, I would love to get in a ring with him, I will expose him," another tweet reads.
Although there is no independent verification of what was said between the two players, Garnett and Villanueva did get into it during the game.
Villanueva was fouled by Garnett en route to the basket with 2:39 to go in the fourth quarter, after which the two exchanged words. Celtics coach Doc Rivers immediately subbed Garnett, but the two were still jawing as he was headed to the bench. At that point, a double technical was called.
"It was something that went beyond trash-talking, in yesterday's game," said Robert Villanueva, Charlie's older brother and manager. "Charlie's been teased all his life regarding his skin condition. He's the spokesperson for the National Alopecia Areata Foundation and he has made anti-bullying in public schools across America his foundation's primary initiative and purpose. It's one thing to hear negative, insulting comments from sports fans rooting against you, but to hear it from your peers it's just complete stupidity."
Robert Villanueva did take exception with his brother's reaction, but he also understood.
"I think Charlie could have handled it a little differently, but he's passionate about his cause and beliefs, and I don't blame him for lashing his frustration with his Twitter community," he said. "Keep in mind, we just completed a cancer awareness month and the NBA league is a full partner in fighting this cause. I just don't get it."
Nuggets coach George Karl, himself a cancer survivor, reacted to word that Garnett had called Villanueva a cancer patient.
"Sometimes, responsibility comes from knowing when to argue when not to -- when to cross the line and when not to cross the line," he said, according to CBSSports.com. "So philosophically, if Kevin and I were close friends, I'd probably call him up and say, 'I don't think that's right [what you said].' But I also believe that competition makes us do things when we don't have our total mental morality in line. We act like children at times, even coaches."
Rivers was frustrated by the incident.
"I actually heard what Kevin said, I was standing right there, and what he released [in a statement through the Celtics organization] is what he said," Rivers said before Wednesday's game. "I'm going to leave it at that. I don't like the whole tweeting thing, I'll state that as well. I think guys talk on the court, that doesn't mean they should or shouldn't. But the fact that we're talking about this, to me, is just silly. It really is.
"We should be talking about [how] we had a hell of a game the other day and let's talk about basketball. It's amazing to me that this stuff is news now, that's not sports."
Information from ESPNBoston.com's Chris Forsberg and ESPN.com's Justin Verrier contributed to this report.