The former Barcelona keeper was shunned from the first team by manager Louis van Gaal after making just two appearances.
Valdes, who joined United on a free transfer in January last year after leaving Barca in 2014, spent the majority of his days training away from the senior squad with academy goalkeeping coach Alan Fettis.
In an interview with El Pais, Valdes said: "What if I were to hit the post and were to lose consciousness, and if I swallowed my tongue?"
"Alan replied: 'I've got my phone in my pocket, I'd call and they'd come to help you.' I told Alan: 'I feel very alone.'
"I was training alone [at United]. Do you know what is that? Here [at Standard] I am happy just to be in the dressing room. For me [Standard] are the best club in the world."
Valdes spent 12 years in Barcelona's first team.