Here's TheFinalPoint.com's breakdown of the main card:
Brendan Schaub vs. Gabriel Gonzaga
Heavyweights Schaub and Gonzaga will kick-off the UFC 121 pay-per-view card and it's without question, the biggest fight to date for Schaub.  A win over a savvy veteran like Gonzaga, would be a big step in the right direction for the former NFL player who's coming off back-to-back knockout wins. 

Web sites and radio shows across the country speak about Gonzaga as if he's an old washed up bum, but the Brazilian has shared the Octagon with some of the UFC's biggest names. 

Overreaction is running rampant over the fact that Gonzaga's dropped three of his last four fights.  What's being failed to be mentioned is that those two losses were to Shane Carwin, who was arguably one punch away from knocking out current heavyweight king Brock Lesnar and Junior Dos Santos, who is next in line for a heavyweight title shot. 

Both Gonzaga and Schaub hold first round knockout wins over Chris Tuchscherer, and both fighters have proved they have dynamite in their hands, but at this point in their careers, Gonzaga is the more well-rounded fighter and has more offensive weapons than Schaub.
Prediction: Gonzaga round 1 knockout

Tito Ortiz vs. Matt Hamill
This very well could be the last time we see Tito Ortiz fighting in the Octagon.  UFC president Dana White told the world that he and Ortiz were back to being friends and the nasty things said between the two was water under the bridge.  But it appears now that even White could only put on a fake smirk for so long. 

"Obviously, we're in a pay-per-view business, but this is a real sport," White told MMAJunkie.com when hinting that Ortiz was in dire need for a victory.  "Tito's lost three of his four fights." 

Ortiz has claimed he's been 100 percent for his last three bouts and then immediately offered excuses for his poor performances afterwards.  I don't see any reason we should start believing him now.  Just for arguments sake, even if Ortiz is healthy, it doesn't change the fact that he's fighting a younger version of himself in Hamill.  Sure, Hamill's takedowns aren't as clean and powerful as Ortiz' used to be, but it'll be enough to get the job done.  Ortiz' best years are behind him and even though Tito has five fights left on his contract, it would come as a shock to no one if he were released after Hamill defeats him.
Prediction: Hamill via decision

Diego Sanchez vs. Paulo Thiago
Paulo Thiago, was coined as the "AKA Killer" after a knockout victory over welterweight contender Josh Koscheck and a round two submission over Mike Swick.  Thiago looked very good in both, the Koscheck and Swick fight but was dominated by Martin Kampmann at UFC 115. 

Sanchez is having troubles of his own as he strangely moved up to welterweight after being dominated by then lightweight champ, B.J. Penn.  It clearly seemed like an overreaction after Sanchez got picked apart by the heavy underdog John Hathaway in his move back up to welterweight.  Then there was the issue where Sanchez briefly parted ways with training guru Greg Jackson. 

Sanchez' fighting career was on the ropes after a slew of odd decisions but since the Hathaway loss, he's rejoined Jackson's team and looks to get back to his wrestling roots. 

This fight is a bit of a tossup considering we aren't sure what Diego Sanchez is going to show up and  the same could easily be said about Thiago.  In the end, however, there isn't a better guy out there than Greg Jackson who will give Sanchez a fail-proof strategy to win.
Prediction: Sanchez via decision

Jake Shields vs. Martin Kampmann
Shields is making his UFC debut after being unable to come to terms on a new contract and leaving as the Strikeforce champion.  Shields is feeling extra pressure in the Kampmann fight after "unofficially" being given the nod to be the next in line for a shot at the welterweight title.

Kampmann is coming off back-to-back victories over Jacob Volkmann and Paulo Thiago.  While his opponent, Shields, will be next in line for a title shot depending on the outcome of Saturday's fight, Kampmann was not given the same offer and will have to find an alternate route to the belt.

Although, Kampmann has clean striking and decent takedown defense, he doesn't pack the same punch that Dan Henderson does.  Henderson crushed Michael Bisping with a hook that left the British fighter completely unconscious for several minutes.  Henderson landed the exact type of blow in Shields' final Strikeforce title defense last April, but Shields managed to quickly recover and proceeded to put on a wrestling clinic, grinding out a decision win over Henderson.

Kampmann doesn't possess the same type of power as Henderson nor does he have the same level of wrestling.
Prediction: Shields via submission round 3

Brock Lesnar vs. Cain Velasquez
A true testament of does size really matter in MMA?  Lesnar outweighed Velasquez by approximately 20 pounds at the weigh-ins on Friday and will likely add to his weight.  Lesnar was seconds away from being dethroned after Shane Carwin caught the former pro wrestler and continued to pummel Lesnar before gassing himself out and eventually being submitted by the champ in the second round. 

Many have said Lesnar's standup is a glaring hole in his game and nobody's arguing that his head movement and striking are pretty.  But Lesnar, on several occasions, has proved that he is one of the best in the game when it comes to patching up holes, adjusting and coming into a fight with the right game-plan. 

Velasquez holds the advantage in the striking department and that contest isn't even close.  He possesses the quicker hands and puts combinations together beautifully.  It also shouldn't be overlooked that Velasquez is two-time, All-American division one wrestler. 

Certainly, it wouldn't be impossible for Velasquez to rock Lensar and if he did, Cain has the speed to close the distance and capitalize but it's doubtful that the fight will ever reach that point.  Lesnar is the taller, bigger, stronger guy and at some point, size does matter.  It's difficult to see a scenario where Lesnar takes Velasquez down and Cain is able to escape from underneath the massive Lesnar. Prediction: Lesnar TKO round 2