Having fallen out with manager Louis van Gaal over the summer, Valdes is widely expected to leave United and recently, it had been claimed that La Liga side Sevilla were interested in signing the former Barcelona star.
However, Castro has moved on to refute such claims and has stated that his side have no need for Valdes currently.
"Valdes is not an option. We have no intention of finding goalkeeping cover," he explained.
"We have three or four players for that position and Sergio Rico is the one closest to keeping it for many years. If we have good keepers, we don't have to go looking elsewhere for them."