Crystal Palace had initially submitted a deal sheet to sign Solomon, expecting to finalize the negotiations by the 7pm deadline on Monday.
However, the deal collapsed shortly after, leaving the door open for Villarreal to make the signing. The Spanish team took advantage of the situation, as the local deadline was not until 11pm.
Solomon spent last season on loan with Leeds United and has now joined Villarreal on a straight loan. The LaLiga club will cover 100% of his wages during his time with the team.