Even with Sergio Aguero rested, Balotelli was left on the bench following his training ground bust-up with manager Roberto Mancini.
It was left to Tevez, the source of such controversy at the club last season, and a goal on his debut by teenage Portuguese midfielder Marcos Lopes to put the smile back on Mancini's face.
Lopes, nicknamed 'Rony' after the Brazilian forward Ronaldinho, only turned 17 last week and struck the third in injury time after Gareth Barry had made in 2-0 just before the break.
With such attention on the club, Mancini's line-up reflected a manager not prepared to risk an upset. Only Aguero and keeper Joe Hart were rested, with Javi Garcia coming in for Yaya Toure who is on African Nations Cup duty.
The importance attached by Zola to the FA Cup was perhaps underlined by him making six changes. With the Hornets challenging for promotion to the top flight it is clear where their priorities lie.
Tevez was straight into the action, carving out an easy chance for Edin Dzeko, who was wearing gloves despite a mild afternoon in Manchester, and he fluffed his effort.
Vincent Kompany, a stalwart in defence all afternoon, was less impressive in front of goal when he then passed up another straightforward offering.
The pressure on Watford was unrelenting though and when City took the lead in the 25th minute the visitors had barely strayed into their opponents' half, let alone had a shot.
City had already had a Dzeko goal ruled out for offside when Tevez was crudely felled 25 yards out by Watford skipper John Eustace, who was booked.
Tevez ran up and drilled the free-kick with immense power, the ball still rising as it hit the back of the net with keeper Jonathan Bond clutching at thin air.
Almost immediately afterwards, Watford had - and spurned - their best chance of the match. An excellent passage of one-touch passing saw Fernando Forestieri, one of the many Udinese players on loan at Watford, through one-on-one with City's keeper Costel Pantilimon but the Romanian pulled off an excellent diving save low to his left.
It was 2-0 a minute before half-time as City sliced through Watford's defence again. David Silva's ball to James Milner created the space and his cross was easy for Barry to head firmly past Bond.
The one-way traffic continued in the second half - less than a minute after the restart a sweeping move by City ended with Tevez firing just across goal.
Milner should have made it 3-0 but he stumbled as he was about to shoot and the ball deflected wide.
Watford, who kept their shape well despite the intense pressure, tried to hit back on the break and Matej Vydra launched a drive from 25 yards out but it was deflected over.
Then came Balotelli's chance to make amends. As he waited to come on in the technical area next to Mancini, the Watford fans chanted 'fight fight fight' but he nearly made amends with his first touch, cutting inside and leashing a powerful shot that was well saved and Tevez stabbed the rebound wide.
Balotelli had another chance from a free-kick from about the position where Tevez had struck his scorcher. Tevez let the Italian have this one but it was disappointing, hit straight into the keeper's arms.
With three minutes left Mancini gave a debut to Lopes and it turned into one of the dream variety - the teenager pouncing to fire home in injury time after Bond had only managed to parry Balotelli's low drive.
Source: Manchester City MAD