The Brentford captain made his first return to Leeds United on Tuesday.
But Casilla has already replied to one of Jansson's Instagram posts, reminding the Brentford captain of Leeds Championship position.
Jansson returned to West Yorkshire with his new employers on Tuesday and failed to prevent a 1-0 defeat against the side he left for £5.5 million last month.
And the Swedish international concluded afterwards: "An emotional night and unfortunately we couldn't hold out until the last minutes.
"A good game between two good teams and a good test for us too.
"On to the next one!"
But while a number of Leeds players responded to Jansson's post with messages of warmth, Casilla adopted an alternative approach - leaving the following comment.
Leeds have conceded just twice in four league games this season and boast the Championship's joint-best defensive record, alongside Fulham and Sheffield Wednesday.
Jansson's replacement Ben White has won plenty of admirers since his summer loan move from Brighton and Hove Albion.
Have something to tell us about this article?