Valero is joining Villarreal today.
Di Matteo told the Birmingham Mail: "Borja made it very clear to me that he didn't want to come back - he wanted to stay in Spain.
"He had a fantastic season in Spain and his family didn't really settle in England.
"It's always difficult for a player when they don't settle but, on the other hand, we hope to raise some funds to use in the transfer market.
"Whatever I raise through sales I can bring to the rest of the team.
"At the end of last season I said we'd need to strengthen the spine and that we'd need a centre half, striker and central midfielder.