HOUSTON -- Even if he survives a horrific head injury suffered in the crash of his tour bus, doctors treating Grammy-winning Tejano singer Emilio Navaira III said he faces a long recovery and an uncertain future.

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Slideshow: Emilio Injured In Bus Crash

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Navaira, 45, known to his fans simply as Emilio, was behind the wheel of his tour bus at about 5 a.m. Sunday following a weekend show when it slammed into an interchange barrier on the West Loop near the Southwest Freeway, propelling him through the windshield.

Navaira remained in critical condition Tuesday.

The neurosurgeon who operated on the San Antonio-born Navaira to remove a blood clot that put pressure on his brain warned that the performer "might not make it." Physicians hoped a research technique using hypothermia to lower his body temperature would keep Navaira's brain from further swelling and help his recovery.

Doctors Tuesday began rewarming him, raising his temperature by a quarter-degree Centigrade each hour, a procedure that would take about 16 hours to return his temperature to normal.

"Overnight he was stable and there was nothing new. We're keeping him in a medically induced coma to try to help his brain recover. It's difficult to say how much he will recover. But we are measuring the pressures in his brain and the oxygen in his brain, and that's what's guiding our therapy," said Dr. Alex Valadka, director of neurotrauma services at Memorial Hermann-Texas Medical Center and vice chair of neurosciences at the University of Texas Health Science Center at Houston. "In his case, (nothing new) is good. We're still afraid something adverse can happen."

Even if Navaira pulls through, the effects on his health "can be as bad as not surviving," Valadka said, listing possible repercussions as persistent coma, a vegetative state, weakness or paralysis and difficulty speaking. He also said less dramatic but still severe fallout could be memory loss or ability to maintain concentration.

"There's a whole spectrum of things that can happen," the neurosurgeon said. "I know you guys want some hard answers, but it's just impossible right now to make some definite predictions."

He put the survival rate for those suffering similar severe brain injuries at 65 to 70 percent.

"As I told his family, we're not even going day by day, we're going hour by hour at this point," Valadka said. "We've seen people with his injuries go on to make amazingly good recoveries. But it takes a long time."

Police said while the crash remained under investigation, their preliminary probe showed Navaira was not licensed to drive the 26,000-pound bus his agent said the singer loved to drive and normally did.

"He loved to drive the bus. He would drive it in and drive it out. It was ordinary. He loved to drive the bus. That's all I can say," Joe Casias, Navaira's friend and agent.

The singer was among six people -- all but one of them band members -- aboard the bus and hurt in the crash on Interstate 610 in Bellaire, as they were returning home to San Antonio after playing at Houston club date Saturday night.

Assistant Bellaire Police Chief Byron Holloway said Tuesday Navaira wasn't licensed to drive "a vehicle that size."

The singer's blood will be tested for alcohol but the results likely wouldn't be known for a couple of weeks, he said. Also under consideration was the possibility that Navaira fell asleep.

"Right now, the investigator handling the case hasn't been really able to find anyone to say this happened or not happened," Holloway said.

Of those also hurt on the bus, bass player Rick Vega remained hospitalized Monday with undetermined abdominal injuries, Casias said.

The others, including Navaira's brother, Raul, who sang in the band, were treated for cuts and bruises and released.

A family member told KPRC Local 2 that it was Navaira's fourth accident while driving. He had three in Texas and one in Mexico.

Casias said Navaira's wife, children, mother and other relatives were at the hospital where the singer was in the intensive care unit. Navaira, married twice, has five children ranging in age from 2 to 18.

Vigils Held For Emilio

Several vigils were held in the Houston area Monday night, as fans prayed for the recovery of Emilio.

The Houston chapter of the American G.I. Forum held a candlelight vigil at the corner of Fannin Street and MacGregor Street, across from Memorial Hermann Hospital, where Navaira is being treated.

"Where there is doubt -- faith. Where there is despair -- hope," an organizer said.

Many fans clutched rosaries as they prayed for a miracle.

"I'm just overwhelmed -- knowing that that happened to him," an unidentified fan said.

While Emilio recovers in intensive care, his fans cannot imagine the Tejano industry without him.

"It's shocking to know another Tejano singer is in a tragedy. We're praying for him that we don't lose him like we did Selena," fan Debra Medina said.

In San Antonio, Navaira's 9-year-old daughter only has one wish.

"That he will wake up on my birthday," she said.

Her birthday is on Wednesday.

Navaira and his band, Rio, were scheduled to head to California for tour dates later this week. They've released more than a dozen albums, including "Acuerdate," which won the 2003 Grammy for best Tejano album.

Navairo benefited from the explosion of Tejano into the popular music scene brought on by Selena, who was fatally shot in Corpus Christi in 1995. Some fans and critics described him as the male counterpart to Selena.

In all, he's been honored with more than two dozen Tejano Music Awards.

He gained fame after performing at the first Go Tejano Day at the Houston Livestock Show & Rodeo in 1990. The performance set a then-record of attendance with 51,072 people. He went on to play at Rodeo Houston in 1995 with Selena, as well as in 1997, 1999 and 2007.