"It's been a traumatic year for him, very tortuous, even after his departure from Arsenal, his first year in Manchester," Rueda told Diario Sport.
Sanchez's progress was hampered when he suffered an injury to his left ankle while on international duty in October and had to undergo surgery.
"It almost frustrated his loan to Inter.
"Once he had recovered, the [coronavirus] pandemic happened. I hope it [coronavirus] passes fast because he is a great professional. He has a great training culture and he will maintain a high level at any club that can give him an opportunity. He has a lot of football yet to show at the top level."
Sanchez, Chile's all-time leading scorer, is tied to United until June 2022.
While Sanchez was a big hit at Arsenal, he was not as impactful at previous club Barcelona.
"There are many factors that come together for extraordinary players not to succeed in a given place," Rueda added.
"After being a star and having important roles in other places, arriving at Barcelona and having a secondary role is hard. That has been the case for many players there. Few have managed to adapt quickly to that new role, perhaps Neymar or even Arturo Vidal have shown that intelligence. It's not easy to establish why Alexis did not succeed at Barcelona."
Sources told ESPN in April that Sanchez will stay at Manchester United rather than leave in a cut-price deal in the next transfer window.
While he would be open to permanently moving to the Serie A side, they have reservations about the cost of any move as Sanchez's current deal at United is worth £500,000-a-week.