Argentina striker Aguero was in superb form earlier in the season, scoring 19 goals in 21 appearances before being struck down by a knee injury in December.
Since returning from the resulting five-week lay-off a month ago he has not found the net again, going scoreless in five games - a run that has coincided with a downturn in results.
City have not won any of the games since Aguero has returned, slipping seven points behind Chelsea in the Barclays Premier League and crashing out of the FA Cup to Championship Middlesbrough.
City clearly need him back on song and Zabaleta is convinced it will happen sooner rather than later.
Zabaleta said: "We need Sergio to come back to his best like he was in November, December when he was doing everything for the team - scoring goals, working hard for the team.
"But after being injured for a long time for some players it is difficult to come back to his best, in terms of fitness.
"I am sure Sergio will come back to his best
It is about confidence as well
The strikers need to feel they are in a good moment
Scoring goals is the best thing for them and I am sure as soon as Sergio scores one goal then he will again be one of the best strikers in the league."
City's latest setback came against relegation-threatened Hull on Saturday, when it needed a last-gasp James Milner free-kick to salvage a 1-1 draw.
Zabaleta acknowledges the champions' problems are not just confined to the strikers, with City having not kept a clean sheet since they overcame Crystal Palace 3-0 on December 20, 10 games ago.
The 30-year-old said: "We have to say defensively we are not doing great at the moment
We are conceding goals every game
We know clean sheets are always important for the team winning games
"There are defensive and offensive things we need to improve if we are to win something this season."
City need to stop the rot to prevent Chelsea marching further clear but they face a difficult return to action at Stoke on Wednesday.
The Potters, managed by former City boss Mark Hughes, are in good form having won six and lost just one of their last nine while they also won at the Etihad Stadium earlier in the season.
Zabaleta, who was signed by Hughes in 2008, said: "We are under pressure and we know it will be really hard because we know we have to win every game
Everything is very tight and there are no easy games in this league.
"If we want to be a team that is trying to retain the title, and keeps going to the end, we need to win the games."
Source : PA
Source: PA