United confirmed on Saturday that Ferguson, 76, fell ill on Friday and has been in intensive care after undergoing a procedure which went "very well", according to the club.
The Scot managed United for 27 years until 2013, enjoying an immensely successful career in Manchester which saw him win a host of trophies and discover many great players.
Arguably chief among the talents he developed was Ronaldo, whom Ferguson signed from Sporting CP in 2003 as a replacement for David Beckham.
Ferguson nurtured Ronaldo at United and the attacker won the first of his five Ballons d'Or under his tutelage at Old Trafford in 2008, having also claimed his first Champions League title the same year.
Ronaldo was allowed to leave United - like Beckham previously - for LaLiga giants Real Madrid in 2009, but Ferguson was still held in high regard by the powerful forward.
And he is hoping for good news for Ferguson following his illness.
Ronaldo wrote on his official Twitter account: "My thoughts and prayers are with you, my dear friend. Be strong, Boss!"