Johnson, 24, was quietly released by City in December and has now revealed he has been receiving treatment at the Priory Clinic, which specialises in mental illnesses, for some time.
Johnson, a dynamic midfielder once tipped as a future England international and linked with a big-money move to Liverpool, fell by the wayside after being hit by a succession of injuries.
A loan move to Leicester was cut short a year ago and City reached a settlement over the remainder of a contract which was due to expire this summer.
Johnson last played for City in October 2009 and so never featured under Mancini, who took charge two months later, but the Italian is saddened by his plight.
Mancini said: "Michael Johnson was a guy with a big talent and I am sad for this, sad for him. He could do everything with his talent. I am very sorry for him. I hope he can resolve all his problems."
Such was Johnson's decline that he was nowhere to be seen when City won the Barclays Premier League last May and his removal from the first-team squad's web page before Christmas went unnoticed.
City have moved on considerably since the days when Johnson was regarded as the club's future.
Just over three years ago Vincent Kompany was not a regular in the side and was often deployed as a midfielder.
Now he is the heart of the defence and an inspirational captain.
His success this week in appealing against his sending-off in last Sunday's defeat at Arsenal was greeted by much relief at the Etihad Stadium.
A three-game suspension would have significantly weakened Mancini's central defensive options at a time when Kolo Toure is at the African Nations Cup and Micah Richards is injured.
Mancini said: "I am very pleased because I think it is correct, like this. The referee managed the game very well but, in that situation, probably he didn't see great.
"From his position he thought it was a red card but I think that tackle was not a red card.
"Sometimes it probably could be. In England, a tackle is an important moment and it is clear that there are some players that do a bad tackle and deserve a red card, but not in that case."
Kompany's dismissal was the only negative from an otherwise impressive 2-0 win at the Emirates Stadium that kept Manchester United's lead at the top of the table to seven points.
City now have the chance to apply pressure to their rivals as they take on Fulham at home a day before United face a tough test at Tottenham on Sunday.
Mancini said: "Fulham are a good team, they have good players. When we played them away, we won the game in the last second.
"We played very well and deserved to win but Saturday will be a different game. They won their last game away against West Brom, they are very dangerous on counter-attack.
"Their manager has big experience and he knows he can do well. It depends on us."
City could be boosted by the return of star striker Sergio Aguero for the clash with Martin Jol's men.
The Argentinian has missed the last two games after tearing a hamstring in the New Year's Day win over Stoke.
Forward Samir Nasri is back from suspension and midfielder Jack Rodwell, out since October with the latest in a series of hamstring problems, could be on the bench.
Mancini said: "I am happy because I am sure Jack will be an important player for us in the future, because he is young.
"If we have solved his problem - his hamstring injuries have been often - he will be an important player.
"(On Saturday) he will be ready, but probably for 20 minutes, no more."
Midfield linchpin Yaya Toure is also on international duty but Richards and fellow defender Maicon are now the only injury concerns.
Mancini has again downplayed speculation he could strengthen his squad in the current transfer window.
He said: "In January it is difficult to take good players.
"Maybe we can find young players. If we have this chance in the next 10-12 days, probably we can do something but I am not sure."
Reserve team striker John Guidetti, who had a successful loan spell at Feyenoord last season but has been hampered by leg problems relating to a virus this term, has been linked with another temporary switch.
The Swede did score for City's elite development squad this week but Mancini does not expect him to move out again.
He said: "Guidetti is not ready to play in this minute. It is difficult for him to recover after his injury. I think he needs another two months."
Source: Manchester City MAD