Frank Mir has first hand experience when dealing with the mammoth wrestler in Brock Lesnar. He went 1-1 against Lesnar, and while he doesn't like the guy that much, but he still picks him to win at UFC 141.
Mir suggests that Alistair Overeem won't be able to stop those takedowns. He also says that trying to time those shots with knees won't work against a guy like Lesnar:
If Overeem sprawls or tries to knee, it'll do him no good.
"Brock doesn't shoot that low. He'll put his forehead in your chest, and he's going to run you through. If he's not able to take you down with that initial blast, he's going to run you against the cage. Then he's going to rip your legs out from under you. Now, you should have gotten taken down in the middle of the area, but instead, you've been taken down against the cage [where it is more dangerous]. Speaking from experience, that sucks."
Mir, was a high school state wrestling champ and a BJJ blackbelt, but that didn't help him avoid those takedowns, or survive on the ground. Even more accomplished wrestlers like Shane Carwin and Cain Velasquez got taken down as well.
Velasquez used his wrestling and was able to get back to his feet long enough to overwhelm Lesnar standing, Mir though, isn't convinced that Overeem can repeat that feat:
"Now, you have a guy who is not a black belt in jiu-jitsu and who is not a wrestling All-American who is going to be able to get back to his feet, so what does he do?" Mir said.
Are you going to try to tell me he's going to wrestle with a national champion wrestler for five minutes? His cardio is going to suck [after wrestling with Lesnar for a while]. I've seen guys with muscles like that, and that's why he slows down