As far I can see, you already understood the saying since you did mention:
They are talking about why a 'too green' employee should not be left alone.
Which is true, but the thing is that in Spanish, the same words can mean different things depending on context.
So, with this, a relative translation to English, using the context you used (a new or unexperienced employee at his job) would be:
He can't do it by himself yet. He sees everything as the most difficult thing ever.
He as no experience, therefore he can't do it himself.
He's not able to do anything by himself right now. It's hard for him.
Hope this helps you, and about your comment I'm not pretty sure that if it's related, since the saying itself is like a description of a person not being able to do something due to his lack of experience and this causing him to see every labour too difficult. Greetings!