Valencia - a £12 million signing from Pachuca at the start of the 2014-15 Premier League season - was linked with a move to Chelsea during the January transfer window.
However, a deadline day transfer never materialised and the Ecuador international has struggled, only scoring twice since November.
West Ham are set to host Premier League leaders Chelsea on Wednesday and Allardyce has blamed the club for turning Valencia's head.
"I think [the speculation] affects you without doubt, subconsciously," said Allardyce. "I don't think he generally goes around thinking it has affected him as no player ever tells you that they are playing badly.
"But, subconsciously, it has to have an effect as you are not absolutely certain about your future.
"I think that there has definitely some thought that goes on subconsciously in terms of, 'Where am I going to be next year? Should I stay here or should I not? Should I go somewhere else? Or I am just looking after myself as it [the deal] is already done?'"
