It has been reported that Italian club Lazio made a £10 million offer for Valencia.
According to The Daily Express, West Ham rate Valencia at around £13m and are unwilling to settle for anything less.
It has been reported that the London outfit signed the forward for £12m from Pachuca in the summer of 2014.
The Ecuador international scored four goals and created 12 chances in 19 Premier League appearances for West Ham last season.
Valencia had a pass accuracy of 75% and a shot accuracy of 48%, and won 23 headers, 15 dribbles and 13 tackles.
During the 2014-15 campaign, the forward scored four goals and created 17 chances in 32 league matches for the Hammers.
The 26-year-old had a pass accuracy of 77% and a shot accuracy of 47%, and won 41 headers, 39 dribbles and 24 tackles.