Tim is a lot better than you're making him out to be, and I don't think you pinpointed his "weakness". The only people to stop him on the ground were:
Frank Mir: Mir didn't get Tim to the ground with a take down, Tim took him down and got caught in an armbar
Arlovski: Ended with an achilles hold after a knock down punch
Randy: Randy won with take downs, but even a 6 time champ in Couture went 5 rounds with Tim and couldn't put him away.
Nog: Tim made a horrible choice and neglected to keep the fight standing after beating Nog's ass the whole fight.
Fedor: Again, submitted after being knocked down in a stand up exchange.
Tim beat Monson to a pulp and his abu dabi credentials are top notch. Ricco looked invincible when Tim ran through him (Top 5 on every list you could find) and to this day is one of the best Heavy Weight BJJ practitioners in the MMA game, he didn't do too good of a job getting in close and taking Tim down.
I've gone to a bunch of live UFC's and have been a member of Sherdog (the biggest MMA community) since 2004, way before The Ultimate Fighter tv show brought in a bunch of know nothing retards.
I'm worried about Randy's age and his ring rust, nothing else. The odds of Lesnar winning a stand-up exchange are slim to none, and he isn't going to pull off any subs with 2 years of BJJ under his belt.
Randy has had mixed results with super sized heavy weights in the past with his losses to Ricco and Barnett, but both of those have some notes worth mentioning...
Randy kicked Ricco's ass up and down for 4 straight rounds before Ricco got on top of him and broke his orbital, and Barnett tested positive for juice when he beat Randy and hasn't returned to the UFC since.
Where does Lesnar win this fight? If history tells us anything, Greco Wrestlers (Randy, Dan Henderson, Matt Lindland, Rulon Gardner, etc.) have had MUCH more successful transitions to MMA than freestyle wrestlers like Lesnar.
I'm not saying its impossible, but smart money is on Captain America.
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Hi, my name is Mark, and I collect (and now sell!) Prototypes.