A ringer
A ringer is that one guy who comes in and makes your team look like a bunch of kids who just started playing hockey.
"That guy is a ringer. He's got the skill of a pro, and he's only here to make us look bad."
"He's the ringer. We're all amateurs, and he's got the talent of a pro. He's only going to play a few games before he disappears."
"He's the ringer. We're all going to lose, and he's going to get all the credit."