an entrenador de base is someone who teaches basics and fundamentals to kids or also professional atheletes, in Spanish:

La labor principal de un entrenador de base es la de educar a los niños en una serie de valores, tales como la solidaridad, el compañerismo, la amistad y el ...
Algunas de las cosas, que entiendo como mucho más importantes que saber de táctica y técnica, y que un buen entrenador de base

so it is someone who teaches the fundamentals and basics of the game

this is someone who is usually a coach as opposed to a manager or head coach, so examples in English:

NFL Football Skills Coach / Sports Performance Coach.
If ever there was a call to arms for multi-skills coach this is it.
As your sport psychologist or mental skills coach there are few facts about sport psychology, performance psychology, and mental skills training that could help ...
John Coumbe-Lilley said the best part of his role as "Sport Psychology Consultant/Skills Coach" for the USA Women's Rugby ...
Former All Black named Blues skills coach
James Howiantz. Somerset. James is an FA Skills coach with the Somerset team.

or "managers in basement divisions"

Laureana, if your link is the actual article you are translating, then from the context it seems to refer to a manager of teams in lower divisions or lower age categories:

"Y ha pasado de la noche a la mañana, de técnico de base a la Bundesliga. Tras un año con niños, pasó a dirigir al TSV Havelse, de la Regionalliga Nord y el pasado ejercicio tomó las riendas del modesto Paderborn 07, al que ha ascendido a la máxima categoría. ¡Y ahora es entrenador Bundesliga!"

This implies that the "técnicos de base" are managers of full teams (as opposed to just training a specific area) in lower divisions of the league. My reading of it.

Aspirar a ser entrenador profesional no es sencillo y el camino por recorrer es duro. Aunque todo radica, como en el caso de Breitenreiter, en que confíen en los técnicos de base.

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Note added at 1 hr (2014-11-30 18:54:30 GMT)
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Laureana - to be honest I've never heard of the term before, I'm just going on your link.
I don't think a "técnico de base" forms part of the training/management team, he/she IS the general trainer/manager but of a not-so-important team, as the team is only semi-professional (not top division anyway) or still quite young.
I hope it helps but it's not very concrete.

Another possibility. For people who train kids, which is what the article is referring to, I think.

http://mhysc.org/Default.asp?id=club-news&l=1&a=article&cid=...

washingtonpremiersoccer.com/forum/index.php?topic=22601.10;wap2




Dec 9, 2013 - Everywhere else in the world, soccer is the number one sport ... The gap between the top of US Soccer and the grass roots coach is wide and ...

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Note added at 1 hr (2014-11-30 19:06:25 GMT)
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Blowing my own trumpet!!

http://www.proz.com/kudoz/spanish_to_english/sports_fitness_...