A report from the Yorkshire post claims that Casilla is expected to arrive in England early next week to undergo a medical and will complete a permanent move from the European champions.
Casilla, 32, has 18 months remaining on his Real Madrid contract, and Leeds are reportedly happy to buy him out of it after he fell to third choice at the Bernabeu due to the arrival of Thibaut Courtois in the summer.
Casilla, 32, is now behind Thibaut Courtois and Keylor Navas in the pecking order at the Bernabeu.
United wanted to loan Darlow from St James' Park but were put off by Newcastle's £4m asking price.
Casilla made his first impression at senior level with Espanyol, a club he joined from Real on a free transfer in 2007 and played for over 100 times. After playing 115 LaLiga games for them, he returned in 2015 after Madrid sold similarly named but significantly more legendary keeper Iker Casillas to Porto.
Barring any hitches, the deal should be confirmed in time for next Saturday's clash with Stoke City, giving Bielsa competition for existing number one Bailey Peacock-Farrell.
Confirmation of a new keeper will turn United's focus to the acquisition of a new winger and a new attacking midfielder, two positions which the club identified for strengthening before the window opened.