Wikipedia describes a knowledge worker as a worker whose "main capital is knowledge". Examples include engineers, doctors, accountants, software writers etc.
Is there an "established term" to refer to non-knowledge workers?
The man who coined the term knowledge workers differentiated them from manual workers.
Management guru Peter Drucker coined the term "knowledge worker." In his 1969 book, The Age of Discontinuity, Drucker differentiates knowledge workers from manual workers and insists that new industries will employ mostly knowledge workers.
Blue-collar worker (in the U.S.) is what comes to mind.
I would say there is no opposite of "knowledge worker" just as there is no opposite to "accountant". Besides, the term "knowledge worker" lacks a precise or agreed upon definition, so it should not be a surprise that there is no clear opposite.
For example, what would you call an engraver, like the kind who engraves printing plates for the US Mint to print US currency? Obviously he is very skilled, but since he is working with materials and his output is a physical object, he is not considered a knowledge worker. Now compare the engraver to a graphic artist who creates digital art on a computer. The jobs are very similar, but the graphic artist fits most definitions of a knowledge worker. The distinction between the two jobs is pretty minor.
I believe the point of coining the term "knowledge worker" was to highlight the fact that whole new classes of occupations were arising and would become a large part of our economy, and the entire dot com boom proved that to be true. But even "unskilled" labor such as picking cotton requires some amount of knowledge and even the most sophisticated "knowledge work" requires some amount of manual labor. "Knowledge worker" is a pretty slippery term.
The opposite of knowledge worker would be an unskilled laborer.
Investopedia defines it:
A segment of the work force associated with a low skill level or a limited economic value for the work performed (human capital). Unskilled labor is generally characterized by low education levels and small wages. Work that requires no specific education or experience is often available to workers who fall into the unskilled labor force.
In the context of labor, skill, according to wikipedia, means:
a measure of the amount of worker's expertise, specialization, wages, and supervisory capacity.
In a continuous view of the workforce, knowledge workers would be at one end, and unskilled laborers would be at the other end.
I came here for an answer, but was not satisfied with what I read, so I will offer an answer. Consider the following types of jobs:
1) The process and the output are clearly specified. (assembly line worker) 2) The output is specified, but the path to get there varies. (a repair tech) 3) Both the process and the output are not clearly specified. (a researcher, a software developer)
The further you get towards #3, the more of a knowledge worker you are.
From this sense, the term "manual" worker is not too bad, but I would still like to see some more alternatives.
Some of the answers to this question are reacting to a value judgement that isn't necessarily present in the term.
A knowledge worker is someone who primarily handles information in their work, in contrast to someone who primarily handles physical materials. There's no judgement being made on skill or value, even though the original author's "other" type of worker was referred to as a manual worker which sounds somewhat judgemental.
They are knowledge workers not because they need knowledge to do their work, but because their work is to operate on knowledge, transforming it etc. It is the material on which they work that they are named for.
A case could be made for some doctors — surgeons for example — being classed as 'manual' workers.
(edited to add third paragraph, clarifying distinction between need / operate on)