Sergei, I think, can still be something. He looked awful against Monson, no doubt, but he can learn from that (Like, get better wrestling and a better ground game).
He got his ass kicked in his K-1 fight, but he took on a stud, and still made it far.
If he fought Barnett right now, though, he'd get subbed.
Sergei should look into Overeem's "horse meat" diet.
Sergei got submitted by one of the best grapplers in the entire HW division, cut him some slack.