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.