Brock Lesnar’s latest bout with intestinal illness changed everything for Carwin, the 36-year-old mechanical engineer from rural Colorado. Instead of easing into the UFC shallows, he’s jumping right back into the deep end against Brazil’s Junior Dos Santos (12-1), widely considered the world’s No. 2 heavyweight.

When Lesnar dropped out of his fight with Dos Santos, Carwin (12-1) stepped in for the main event of UFC 131 tonight in Vancouver’s Rogers Arena, with a fall title shot going to the winner.

“You get very few opportunities like this in life,’’ Carwin said. “I get to go in there and make the most of it and do something that I love to do. When I got that call, it was an absolute yes. Not only do I get to fight one of the best guys in the world, but it’s with the title shot on the line.’’

Carwin was one punch away from winning that title last summer when he battered Lesnar for nearly every minute of the first round of their bout. Lesnar spent the round curled in a corner of the cage, blocking punches and hoping Carwin would tire himself out.

Carwin did just that, and Lesnar finished him in the second round.

While Carwin dealt with neck pain that led to surgery in the ensuing months, he also realized he had to change his conditioning to compete with Velasquez or Dos Santos, two prodigious natural athletes.

“My diet is mostly organic now, a lot of fruits and vegetables,’’ Carwin said, praising his nutrition consultant, Josh Ford. “We get some chicken and fish in for protein, but he completely changed my thoughts on nutrition and everything we do is for recovery.

“Every time you come out of a fight, you always try to make yourself a better fighter. There were a lot of things that I learned in that space.’’

MMA-mad Canada is hosting its latest UFC show just one day after the Canucks hosted Game 5 against the Bruins in the Stanley Cup Final in the same downtown arena. Veteran featherweight Kenny Florian of Dover, Mass., will meet Brazil’s Diego Nunes, and lightweight contender Donald “Cowboy’’ Cerrone will take on Vagner Rocha before a bout between middleweights Demian Maia and Mark Munoz.

Dos Santos was looking forward to a bout with Lesnar after appearing together on the most recent season of “The Ultimate Fighter,’’ the UFC’s reality show. The UFC also was disappointed when its biggest pay-per-view star had another setback with diverticulitis last month, eventually undergoing surgery to remove a portion of his colon.