And with the club's de facto director of football Dane Murphy also set to be lured away, the Reds face a rebuild after a 2020-21 which exceeded expectations.
Ismael emerged as the Baggies' preferred option on Friday after interest in former Sheffield United manager Chris Wilder and and ex-Huddersfield Town coach David Wagner failed to develop into anything more concrete.
With a £2m release clause in his Oakwell contract, securing his services was always going to be an easy task if the former Bayern Munich and Crystal Palace centre-back was interested.
Agreement is expected to be reached on Monday.
The club's chief executive, Murphy, a former Major League Soccer scout and technical director of Real Salt Lake and DC United, is thought to be close to joining Championship rivals Nottingham Forest, along with Taymour Roushdi.
Murphy was hugely influential in the model which saw Barnsley target unheralded foreign coaches such as Daniel Stendel, Gerhard Struber and Ismael to oversee a squad recruited on very specific, data-driven lines to play a brand of pressing football.
Struber dramatically saved Barnsley from relegation in the restarted 2019-20 Championship season but started last season sowly amid frustration at what he perceived as a lack of boardroom backing in the transfer window, and with interest from New York Red Bulls, who triggered his release clause in October.
Barnsley were winless when Ismael succeeded him but he took them into the Championship play-offs with a brand of football that was extremely direct and not to the liking of West Brom's hierarchy when they started to search for Sam Allardyce's successor.
But the power struggle which led to the departure of technical director Luke Dowling and the difficulty in landing other candidates saw a change of heart and the Baggies, relegated from last season's Premier League, approached Ismael after all.
Barnsley's model, never mind the success Ismael enjoyed, means they will have identified possible successors long ago, but acquiring the services of managers from the lower-profile leagues - Ismael and Struber's previous jobs were in Austria - is harder now post-Brexit regulations are in place.
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