The Northamptonshire all-rounder, who was forced home early from down under, is suffering from a hot spot in his back which showed up on a scan.
And, while he will need a period of time for the injury to heal, he is expected to be ready for the start of the season in April.
It is a double blow for Willey who had to pull out of a deal to play for Auckland in New Zealand’s domestic Twenty20 competition and has been unable to push his case for inclusion for the England Lions’ tour to Sri Lanka in the new year.
It has all been far from ideal. However, head coach David Ripley is pleased all the uncertainties have been cleared up.
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He said: “We’ve managed to get to the bottom of the injury and while it took us a while, I’m glad it has happened now and not been left to drag on.
“The early signs were that it was a stress fracture in an awkward part of his back but a different quality of scan found the problem.
“It’s obviously been frustrating for David who has had injections and some rest.
“He was bowling outside in Australia but it wasn’t going away so we had to change our approach and that, thankfully, has worked.”
As for when Willey will be able to return to the fray, Ripley said he should be able to build up gradually over the next few weeks.
He added: “He’ll take it easy, probably until the middle of January, and he could be bowling by the end of February.
“We then go to Barbados for pre-season and I fully expect him to be bowling properly by then.
“If that all goes to plan, then he’ll be fine for the first game in April.”