The Spanish goalkeeper didn't cover himself in glory for Leeds United on Saturday.
United lost 1-0 at Charlton Athletic on Saturday and a number of visiting players didn't cover themselves in glory.
But the same is true of the Leeds' goalkeeper, who parried a Tom Lockyer volley into the back of Macauley Bonne's for the decisive goal of the game in London.
Casilla joined the Whites from Real Madrid in January and has been guilty of quite a few hairy moments.
Yesterday certainly qualified as another and although Whelan concedes that the big Spaniard can be a weakness for the Yorkshire club, he noted that making mistakes for a goalkeeper is par for the course.
Whelan is right in saying that mistakes are inevitable if you're a goalkeeper, and they are, but when you've made so many in a relatively short period of time then perhaps it suggests that it's more about the player himself rather than the position in which he plays.
Bielsa has other options in the form of Illan Meslier and Kamil Miazek, the former of which is on loan from Lorient in France.
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