Speaking to The Sun, the Portugal international explained how his experience of playing with the striker at United had made him realise how important Tevez is to whatever manager is able to pick him.
"I played with Carlos and he is an important player for any club," said Nani.
"I don't know if he will stay at City but, if he does go, it will be a big problem for [Roberto] Mancini because of the way that he plays and his experience."
The 24-year-old was also keen to point out that the Red Devils are not taking the Community Shield against their cross-city adversaries at Wembley lightly, stating: "The Community Shield is the first official title of the season and we want to win. And facing City makes the match more exciting.
"It's also our first derby of the season, so to lift the trophy would raise our morale and inflict pain on our rivals."
Nani’s own Manchester United future was questioned after his surprise omission from Sir Alex Ferguson’s starting XI in the 2011 Champions League final against Barcelona, but the player says he holds no grudges over any decision that was taken that day.
He continued: "That's all in the past. I've enjoyed good sensations there and prefer to remember those.
"It was the coach's decision and I accept his orders - although, of course, I wanted to get on the pitch."
When asked about the potential of a two-horse title race between United and City, Nani said: "City won't be our only rival. They have reinforced but the competition is very open and it is impossible to discard the other big clubs.
"The Premier League is much improved. The quality is better and each league match is almost like a cup final.
"We must maintain a high level from the very first day. Year by year it gets more difficult to clinch the title - but this is our objective."