The Hornets prop up the Premier League after their ninth loss in 15 games at Leicester.
Foster assessed: “I think it started before the FA Cup final.
“If you look at the finish to last season there was half-a-dozen or eight games where the standard really dropped.
“We struggled to address it at the time.
“It's almost a symptom of being a middling club in the Premier League.
“When you get to a certain point in the season when it's handbrake up, holidays – all that kind of stuff and I think that has creeped in.
“It's almost been left to fester and rot too much and now it's too much of a problem.
“I know the ability we have in that dressing room, I see it on the training pitch, but it's transferring it onto the pitch that counts.
“If you are mentally not right or not confident enough then you can't go and show what you can do."