A second-half hat-trick from Sergio Aguero prevented the hosts from a shock loss to Watford, who held a 2-0 lead at the break, with the manager hailing his side's reactionManuel Pellegrini was "worried" Manchester City would exit the FA Cup as they produced a 'Jekyll and Hyde' showing against Watford.
The City manager has overseen only one defeat at the Etihad Stadium this season, against Bayern Munich in the Champions League.
However, that impressive record came under threat against Watford, with the visitors moving two goals ahead before a second-half resurgence secured a 4-2 victory - with Sergio Aguero netting a hat-trick.
"I was very worried at half-time because I talked with the players a lot before the match and said that, in the FA Cup, teams that play in a lower division have a lot of motivation," he told reporters.
"If you don't play with 100 per cent intensity and desire you can lose and go out of the competition and we were jogging in the first half - if you play like that, you can't win.
"I didn't underestimate Watford; I didn't think it would be easy and I told the team this. The players must not believe they have an easy game but I feel that they had a very good reaction in the second half.
"We changed everything; we played with tempo, with pace. We continued playing in the same way we've played in this whole season at the Etihad. This reaction was very important.
"The whole team didn't do the things that we normally do in the first half, the most important thing for me is that Watford didn't score the third goal. If they'd have scored that, the game would have been finished."