Maurizio Sarri said he took Callum Hudson-Odoi off against Watford because the youngster was struggling with a hamstring problem.
Hudson-Odoi came on for the injured Pedro in the first half but was himself substituted late in the game.
“He played very well for 55 minutes,” boss Sarri explained after the 2-1 win at Vicarage Road.
“He had a hamstring problem in the last week and with 10 minutes to go he had some pain, so we decided to change.
“It was too dangerous (to take risks with him). There wasn’t another solution.”
Eden Hazard scored both the Blues’ goals as they moved back up to fourth in the Premier League table.
It was the response Sarri wanted following the recent shock home defeat against Leicester.
Sarri added: “Against Leicester the reaction from the negative situation was wrong. Today we had much better application for the 90 minutes.”