Looking for a synonym for "experienced worker"

Solution 1:

Such a person is often referred to as an old hand.

From ODO:

old hand: a person with a lot of experience in something.

"the examiner is an old hand at the game"

synonyms: expert, past master, virtuoso, master, wizard, genius, artist, adept, professional, doyen, veteran, maestro, connoisseur, authority, grandmaster, master hand, skilled person

Solution 2:

I honestly think you hit the nail on the head with veteran.

As a noun:

"A person who has had long experience in a particular field."

For example, "We'll have John train you, he's a veteran with the company."

Alternatively you could use it as an adjective:

"Experienced through long service or practice; having served for a long period:"

For example, "John can train you, he's our most veteran programmer."