In his debut in the UFC, he had an impressive win over Justin Eilers with a KO in 3:34 of the first round for his debut fight in UFC 51: Super Saturday on February 2, 2005. His next fight was against Kevin Jordan on UFC 53: Heavy Hitters on June 4, 2005 where Buentello won with a neck crank submission at 4:00 of the first round. He lost a title shot against the heavily touted #2 ranked Heavyweight fighter in the world Interim UFC Interim Heavyweight ChampionAndrei Arlovski at UFC 55 on October 7, 2005, Arlovski knocked Buentello out in 15 seconds setting a UFC record for the quickest title fight in UFC history. Following his devastating loss to Arlovski, he faced off against Gilbert Aldana at UFC 57 where he won via TKO due to strikes.
Strikeforce
After leaving the UFC, Buentello fought for Strikeforce, winning 3 straight against K-1 Kickboxer Carter Williams, UFC veteran Tank Abbott, and experienced journeyman Ruben Villareal, before losing to Alistair Overeem in a bout for the Strikeforce Heavyweight Championship.
Affliction
Buentello moved to a new major organization, Affliction, where he won his first two bouts. His third fight was set to be against Gilbert Yvel at on August 1, 2009, but the entire event was ultimately cancelled due to complications arising from heavyweight Josh Barnett's inability to receive a California State Athletic Commission licence. Affliction Entertainment became defunct as a Mixed Martial Arts promoter shortly after.
UFC return
At a press conference held July 31, 2009, it was announced that Buentello was one of several former Affliction fighters to sign with UFC. Buentello was expected to face Todd Duffee at UFC 107, but Duffee was forced off the card with an undisclosed injury. Buentello instead faced 6'11 Stefan Struve and lost a majority decision. Subsequently, Buentello lost to Cheick Kongo on where he surprisingly—after a prolonged period of having his legs pounded—he tapped due to elbows to the right thigh in the third round. After losing both bouts in his return to the UFC, Buentello was released from the promotion.
Independent promotions
Buentello continued his MMA career at the Texas-based Shark Fights promotion, defeating the previously unbeaten Bryan Humes via Unanimous Decision. Buentello fought former UFC Heavyweight ChampionTim Sylvia at Powerhouse World Promotions: War on the Mainland on August 14. In a very one sided fight, Sylvia defeated Buentello via KO due to an uppercut in the second round. Buentello was scheduled to face Kerry Schall at on November 13, 2010 but the event was postponed to December 10, 2010 to avoid a tropical storm. He won the fight by unanimous decision. Buentello, like the other MMA fighters on the card, did not receive his contracted pay. His next bout was a non-title fight against decorated amateur wrestler and current Bellator Heavyweight ChampionCole Konrad at Bellator 48 on August 20, 2011. He lost the bout via unanimous decision. On March 31, 2012, Buentello faced former Shark Fights Heavyweight Champion, and The Ultimate Fighter 10 alumni Darrill Schoonover WMMA 1: McCorkle vs. Heden. Buentello lost by unanimous decision. Buentello was scheduled to face Marcus Sursa at Legacy Fighting Championship 22 on August 23, 2013, however, the bout was scrapped after Sursa was denied a license by the Texas Department of Licensing and Registration. He instead faced James McSweeney at the event. He won the fight via TKO in the second round. Buentello faced independent prospect Myron Dennis on March 15, 2014, at Legacy FC 29 for their vacant Light heavyweight championship. Buentello lost the back-and-forth fight by split decision. Buentello fought Rameau Thierry Sokoudjou in the Heavyweight class on October 3, 2015 in Abu Dhabi, UAE at Abu Dhabi Warriors Fighting Championship 3. Buentello won the fight via KO with punches in the third round.
Absolute Championship Berkut
Buentello faced Denis Goltsov at ACB 41: Path to Triumph on July 15, 2016. He lost the fight via knockout in the first round.
