The Italy striker facing a lengthy lay-off with a knee injury suffered at the Santiago Bernabeu on Wednesday, but the extent of the problem is not yet fully known
The Italy international came off in the second half of his side's 3-0 La Liga defeat against Real Madrid on Wednesday, being replaced by Mario Gaspar.
Medical tests have shown that the 24-year-old has a torn anterior cruciate ligament tear in his right knee, with an estimated recovery time of six months.
Rossi is currently at the 9 de Octubre hospital in Valencia, where will be operated on the problem by doctor Enrique Gastaldi on Thursday evening.
The news comes as a heavy blow to Villarreal, who are struggling in the league with just seven points from nine games.
Fellow striker Nilmar has already been out of action for a month, also with a knee injury, and it will take the Brazilian at least another four weeks to get back to full fitness.