Johnson, once touted as a future City star and England international, battled injury and mental health problems during his time with the club and, having not featured for the first-team since October 2009, was quietly let go in December.
Although he never managed the 24-year-old, Mancini felt compelled to express sympathy with the youngster's plight, and voiced his hope that he can now sort out his personal issues.
"Michael Johnson was a guy with a big talent and I am sad for this, sad for him," he told reporters.
"He could do everything with his talent. I am very sorry for him. I hope he can resolve all his problems."City take on Fulham this Saturday as they bid to close the seven-point gap to Manchester United at the top of the Premier League, and Mancini is warning his players that the Cottagers will be no pushovers.
"Fulham are a good team, they have good players," he added. "When we played them away, we won the game in the last second.
"We played very well and deserved to win but tomorrow 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."