Casillas is one player, along with Wales winger Gareth Bale, who has been blamed by Real fans for the European champions' slump and he was again the target of their ire during Tuesday's 4-3 Champions League loss at home to Bundesliga side Schalke 04.
Although they squeezed through to the quarter-finals 5-4 on aggregate, Casillas and his team mates were whistled off the Bernabeu pitch by their angry supporters, already fuming after Real were beaten at Athletic Bilbao in La Liga at the weekend.
"He did not play well and he admitted that," Del Bosque said in an interview with Radio Marca.
"He also saved Madrid at the end," added the 64-year-old, who coached Casillas when he was in charge at Real between 1999 and 2003.
"We don't have to make a change unless the dip in form is alarming. I think he is in good shape."
Del Bosque has plenty of options if he decides to call time on Casillas's international career, with the likes of David De Gea (Manchester United) and Kiko Casilla (Espanyol) pushing for a starting place.
However, he has consistently backed his captain during several difficult periods in recent years and is unlikely to drop him unless he suffers a sustained slump.
Casillas has made 160 appearances for Spain, more than any other player, and led the side to back-to-back European crowns in 2008 and 2012 with the nation's debut World Cup triumph in between in 2010.
Spain's next game is a Euro 2016 qualifier at home to Ukraine on March 27.
With four matches played, Spain are second in Group C on nine points, three behind leaders Slovakia, who beat them 2-1 in Zilina in October, and level with Ukraine.
(Reporting by Iain Rogers, editing by Ed Osmond)
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