Idiom for players / employees that proved to be more talented than expected

A prodigy, a high-flyer, a discovery, a revelation, a wunderkind are all terms that come to mind, depending on the audience to whom you are talking. Of these I prefer the anglicised German, wunderkind, a child wonder.

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:

1. one who achieves unexpected support and success as a political candidate, typically during a party's convention.
2. a little-known, unexpectedly successful entrant, as in a horserace
3. a competitor that is relatively unknown or that wins unexpectedly.

A couple of sentences:

1. 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.
2. Never having competed before in a piano competition, Andre turned out to be a dark horse and placed second, above more seasoned players.

One possible term is Young Turk. It has a restricted field of meaning as it generally signifies a progressive, revolutionary or rebellious member of an organisation or political party, especially one agitating for radical reform. It's not a term that one might use of say a football star, but maybe someone who wanted to shake up the management of football at a high level.

The term came into popular usage after the Young Turk Revolution of the Ottoman Empire in 1908 which restored the constitution of 1876 and ushered in a multi-party electoral system.

It entered as an idiom in American English in the 1920s when it was used to describe a particular class of US senators who challenged the then establishment.

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.

Whiz kid is the first one that comes to my (older) mind. Dictionary.com defines it as:

a youthful and exceptionally intelligent, successful, or influential person in a given field

We use "sleeper" in some parts of academia. If you run a math competition, you expect superstars from magnet schools in big cities. But once in a while you get a kid on a ranch in Wyoming who just blows the test away. The concept is that there was hidden talent and we've woken it up.

Cinderella or Cinderella Story

See: https://en.wikipedia.org/wiki/Cinderella_(sports)

This is:

• Idiomatic
• Used for someone more successful than expected (which may certainly be due to underestimated talent).
• Primarily used in sports
• Often young
• Usually a one-time event, because after they are "revealed" they are unlikely to be underestimated again

I find frequently (specially in my area of work: software development), that such person is called a rockstar.

As mentioned in another answer, the word revelation itself satisfies the latter two criteria.

If we have to include the criterion "typically (very) young", the closest I can think of is a rough diamond or a diamond in the rough. Even here, young perhaps can only be implied from unpolished (which is what really a rough diamond literally is).

TFD(idioms):

a rough diamond [mainly BRITISH] or a diamond in the rough [AMERICAN]

2. If you call someone or something a rough diamond, you mean that they have talent or good qualities which are hidden or not well developed and could be developed more.
Note: A rough diamond is a diamond that has not yet been cut and polished.

When I heard this lady sing, I ran to the theater, and I said, `Chick, I found myself a diamond in the rough.'

Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012

One possibility not yet mentioned is "ugly duckling":

ugly duckling

n.a person or thing, initially ugly or unpromising, that changes into something beautiful or admirable

It has a somewhat superficial connotation, and might be more typically usually used about appearance than performance, but could work in your example situations.

How about the idiom have a hidden talent? From The Free Dictionary:

have a hidden talent: To have a particular skill or ability that few people know one possesses.

Having a hidden talent conveys that one is talented at what it does, but few know about this until it is "revealed", and it cannot be "revealed" more than once.