Valero spent last season on loan at Real Mallorca and now looks likely to return to Spain on a permanent basis, after agreeing a five-year contract with the Primera Division outfit.
A statement on the Villarreal website read: "Barring a few minor details, Villarreal CF and West Bromwich Albion have come to an agreement for the transfer of footballer Borja Valero, who will belong to the Yellow Submarine club for the next five seasons.
"After the signing of the contract and the medical tests, the club will make official the date of the presentation of the footballer."
The midfielder spent just a season with the islanders before West Brom snapped him up for a club record £4.7 million.
Valero signed a four-year contract with the option of a further year but was loaned out to Mallorca after just one season at The Hawthorns.
The 25-year-old impressed on his return to Spain, winning the prestigious Don Balon award for the best Spanish player.