Idiom for players / employees that proved to be more talented than expected
You could also say they're breakout employees/players.
Breakout (adj): Used to describe someone or something that suddenly becomes very well known or successful in a particular type of activity, or an achievement that helps them do this.
Examples:
She was the breakout star of this year's Women's World Cup.
This was the breakout book by one of the most original writers of his generation.
[Cambridge English dictionary]
There is an idiom which may or may not serve your purposes. It is dark horse.
A dark horse could be a politician, an athlete, a musician, or any person who has a bent toward excellence in whatever field or discipline you could name. What makes a dark horse a dark horse is an unexpected win or a surprise performance. Perhaps the odds are against this person winning, though I think people do not give much, if any, thought regarding a dark horse, let alone consider their odds of winning. That's why everyone is surprised when the dark horse pulls it off!
From the Free Dictionary:
- one who achieves unexpected support and success as a political candidate, typically during a party's convention.
- a little-known, unexpectedly successful entrant, as in a horserace
- a competitor that is relatively unknown or that wins unexpectedly.
A couple of sentences:
- Tina, who had never competed before in a race over a quarter mile, proved to be a dark horse in the mile when her coach suggested she substitute for a sick team member.
- Never having competed before in a piano competition, Andre turned out to be a dark horse and placed second, above more seasoned players.
It seems like there are two aspects to this.
For a young star who comes from nowhere, in U.S. sports talk people use "phenom", short for phenomenon (which is also used). Sometimes combined with "rookie" (a new or first-year player), "rookie phenomenon."
For exceeding expectations (or past history) there are "underrated" (which is badly overused in U.S. sports talk), or "punching above his/her weight" (a sports phrase more often used for countries in foreign relations, e.g., Iceland is punching above its weight to be such an influential leader at this UN meeting), or something more literal like unexpected success.