The league's website says Kubiak suffered a TIA, or transient ischemic attack. NFL.com says that's according to sources with direct knowledge of Kubiak's condition. A TIA happends when a blood clot blocks an artery for enough time to affect blood flow in the brain.
The good news -- mini strokes don't generally cause permanent brain damage, but they are a big warning sign for a much bigger problem.
The Texans haven't said anything more about Kubiak's condition and it's unclear how long he'll be off the sidelines at this point. As of now, he remains in the hospital.
The team says Kubiak became dizzy and light-headed Sunday night, causing him to stop and kneel down on his way off the sidelines. He was admitted to a hospital in the Medical Center.
"Our primary concern is of course with Gary's health and well-being," Texans Executive VP of Football Operations and General Manager Rick Smith said. "There have been so many people throughout the city and across the country that have reached out to express their love and support and we are thankful for everyone's thoughts and prayers. Gary is alert, coherent and in good spirits. He is continuing to be evaluated and monitored."
Kubiak, who remain stable, went through a battery of tests at the hospital Sunday night.
The crowd at Reliant, which had been loud all night with a Texans 21-3 lead, went silent.
"All his vital signs are good," Defensive Coordinator Wade Phillips said during a post-game press conference. "He did not have a heart attack, but they're still checking on him."
"He had an episode; he was light-headed and dizzy," Houston general manager Rick Smith said. "He was evaluated by a number of specialists ... he is awake and coherent."
The Texans said Kubiak's family is with him. His youngest son, Klein, sent a Tweet expressing appreciation for the concern from fans: "Thanks to everyone for all of the thoughts, prayers, texts, and tweets. God has a plan and his healing hands are always in motion."
Players reacted to their coach collapsing during a news conference.
"We were all very worried," QB Case Keenum said. "When we went back out, they told us that he was alright, that he was stable. They didn't know what was going on yet. Obviously we were all upset about that but trying to stay focused at the same time."
Brooks Reed said he was in the tunnel on the way to the locker room when he heard people yelling for medical personnel. He said he thought it was a fellow player and was shocked when he learned it was Kubiak. Andre Johnson said the game was affected by the incident, saying sometimes plays took longer to come in when Kubiak was not there.
No other information on Kubiak's condition or details surrounding the incident have been released.
"We have to assess ... obviously, there's a lot of info," Smith said. "Hopefully, Gary will be back with us tomorrow."
Further information on his condition will be provided when it becomes available.
As for the game, Phillips took over as coach. The Colts came from behind to beat Houston, 27-24.
Before the game ended, Colts owner and CEO Jim Irsay posted his concern for Kubiak on Twitter: " Big time prayers for Coach Kubiak...God bless his healthy return and ease his family's worry and concern."
We asked for fans to post their well wishes for the coach on the ABC13 Facebook page, and responses poured in from around the country.
"Get well soon Coach! We need you back on the sideline!" Terry Yarborough said.
"Prayers for Kubiak and his family right now. From a family of Cowboys fans...we are in Houston and we care about our fellow Houstonians," Amy Davis Votaw said. "Praying for answers and a full fast recovery."
The collapse of Kubiak, who has no known public health problems, came just a day after Denver Broncos coach John Fox was hospitalized in North Carolina as he awaits aortic value replacement surgery. Fox, 58, will have surgery in a few days and will miss several weeks while recuperating.
Kubiak has long been known as a top offensive coach, mentoring quarterbacks in Denver under Mike Shanahan and now Matt Schaub - and Case Keenum - in Houston. He was hired in 2006, along with Smith, after the Texans finished a franchise-worst 2-14. Smith spent 10 years with Kubiak while the coach was offensive coordinator of the Broncos. Smith was Denver's defensive assistant for four seasons before moving into the front office for his last six years with the Broncos.
The pair has helped transform the Texans, which began play in 2002, from league laughingstock to contender. The team went 6-10 in their first year and 8-8 in each of the next two seasons. Expectations were high in 2010 after Houston finished at 9-7 for its first winning record in 2009. But the Texans instead fell to 6-10, which led to many fans calling for Kubiak's firing.
His original contract was due to expire after the 2010 season, but owner Bob McNair has stepped up to keep Kubiak and defended him several times amid the bumps. Among recent departures were assistant head coach Alex Gibbs (for Seattle) and offensive coordinator Kyle Shanahan went to join his father, Mike, in Washington.
Last year, the Texans announced contract extensions for both Smith and Kubiak, rewarding them for taking the team to the playoffs last year for the first time. Kubiak's three-year agreement has him under contract through 2014. McNair said at the time he offered Kubiak a four-year deal, but the coach preferred to make it for three.
Kubiak made his mark as Denver's offensive coordinator under Mike Shanahan, winning two Super Bowls. An eighth-round pick out of Texas A&M, he spent nine years as John Elway's backup. He finished his career 4-1 as a starter, all in emergency relief of Elway.
PLAYERS REACT ON TWITTER
Some players from Texans past, Texans present and from other teams took to Twitter to express their well wishes for the coach.
Danieal Manning, Texans safety: "Prayers go out to the Kubiak family. #GetWellKub."
Tyler Clutts, Miami Dolphins FB and former Texan: "Praying for coach Kubiak and his family that everything is ok and he bounces back quickly."
Rashad Butler, Cleveland Browns OT and former Texan: "My prayers go out to my old HC Gary Kubiak!"
Jesse Nading, former Texans LB: "Wow.. Hope Kub is ok. There is no better man in the business."
Vontae Davis, Colts cornerback: "Thoughts and prayers with Texans HC Gary Kubiak."
James Casey, Philadelphia Eagles TE and former Texan: "My thoughts are with Coach Kubiak. I hope everything is ok."
Glover Quin Jr., Detroit Lions safety and former Texan: "Prayers for Coach Kub!! Great man! Hope he is ok!!"
Jon Weeks, Texans LS: "Thoughts and prayers to Coach Kub and his family tonight."
DeVier Posey, Texans WR: "Tough lost but my thoughts and prayers are with Coach Kubiak ! Rest up see you Monday"
Please stay with Eyewitness News and abc13.com for updates and new details come into the newsroom. Live reports begin at 4:30am.
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The Associated Press contributed to this report.