Seven months ago Chelsea cantered to the Premier League title, but then managed only four wins in 16 games this season before Mourinho left Stamford Bridge last week.
On Saturday he watched from the stands as his new team beat Sunderland 3-1 and now he is preparing for his first game back in charge at home to in-form Watford on Boxing Day.
Speaking of the Blues' struggles this term, Hiddink told reporters: "It was frightening for everyone at the club to see Chelsea so close to the relegation zone."
Chelsea are 15th in the table, 20 points behind surprise leaders Leicester City at Christmas and only three above the drop zone.
Saturday's opponents Watford enter the match on a four-game winning streak and lie just one point behind the top four.
Ighalo (12) and Deeney (5) have scored 17 of Watford's 21 Premier League goals this season.