Kiko Casilla preparing for Leeds United debut - is this the start of a long reign as Elland Road's number one?

Both had their reasons for taking the plunge - largely to do with where Leeds could go, as opposed to where they were - but Bielsa himself felt a touch of surprise at Casilla, an elite goalkeeper, descending on England’s second division. “It’s not common that a player of his level chooses to play in the Championship,” Bielsa said.

Casilla has a substantial contract at Elland Road, running to 2023 after his free transfer from the Bernabeu, and he told Spanish media this week that he had “signed for four years and I want to see them all out.” But it is his reputation and stature which will make Leeds think that they have found a number one who could hold down that position indefinitely.

Bielsa said Bailey Peacock-Farrell, the keeper who is about to make way tomorrow, would “play as a starter again in the future” but Peacock-Farrell cannot fail to appreciate the scale of the competition beside him.

Casilla’s experience has come in different forms: a decent haul of La Liga appearances for Espanyol, on Mauricio Pochettino’s watch for a short while, and many years inside the confines of an uber-ambitious club in Real Madrid.

Rotherham’s stadium will not be much of a culture shock this weekend - a modern bowl in the traditions of 21st-century football in England - but the cold weather and the football might be. His debut is a sub-plot in a derby which Leeds need to turn in their favour.

Victor Orta, United’s head of recruitment, did the groundwork for signing Casilla but the transfer received final approval from Bielsa, who managed in Spain at Athletic Bilbao while Casilla was breaking through with Espanyol.

“I think the fans of Leeds United should be proud to have Casilla as their goalkeeper,” Bielsa said. “It's not frequent that you get a player from Real Madrid and it's not common that a player of his level chooses to play in the Championship.

“Players are always very careful when they have to take decisions. They receive a lot of advice from people before taking them. Casilla comes to Leeds because he thinks he’s not making a mistake by coming here. It’s a decision he thought a lot about. English football and Leeds United should be proud that a player from Real Madrid has come.”

For Peacock-Farrell, Bielsa’s decision to start Casilla tomorrow allows him to retreat to the support trenches, much as the 22-year-old would have chosen to stay on the front line.

Peacock-Farrell is at the end of his longest run of first-team fixtures - 27 Championship appearances under Bielsa - and is making way with Leeds a long way into a convincing bid for automatic promotion. Bielsa talked of goalkeepers going through a “long process” of development and Casilla, at 32, has navigated far more of it.

There were peaks for Peacock-Farrell this season, the highest his 90th-minute penalty save in a 1-0 win over Reading, and troughs along the way too; obvious errors against Birmingham City, West Bromwich Albion and Blackburn Rovers. Since Jamal Blackman broke his leg in November, at a time when the Chelsea loanee was about to make his first start in the league, Peacock-Farrell has been on a learning curve which Bielsa had no way of taking him off. A spell behind Casilla, Bielsa hopes, will lift some pressure from Peacock-Farrell’s shoulders.

“I use the same reason to substitute Farrell as I used when I thought about putting Jamal Blackman in,” Bielsa said. “With the performance of Farrell we have two ways of looking at it. He's a goalkeeper with skills and he has the resources to become a starter in an ambitious team but he needs to correct some mistakes.

“Sometimes when you make mistakes you lose a little bit of confidence but at the same time, those mistakes make you increase your work to correct them. When you make mistakes you suffer pressure and we need to learn to live with pressure. It's a long process with many steps and it's a difficult path, especially when you're a young player.

“It's even more difficult when your team is at the top or bottom of the table so I thought this was the right time to put Casilla in as a starter. But I don't think this will stop the improvement of Farrell.

“The skills Farrell has we haven't seen so far in the games but as we see him every day in the training sessions, we know what his skills are and what he’s capable of. I think he will play again as a starter in the future.”

Leeds’ style of play with Bielsa as head coach, so heavily possession-based, can leave their keeper idle for extended periods but Peacock-Farrell holds one of the best goalkeeping records in the Championship. Nonetheless, the naked eye has seen shortcomings and uncertainty which manifested itself in choice exchanges between Peacock-Farrell and Bielsa’s centre-backs, Liam Cooper and Pontus Jansson, in last weekend’s defeat to Stoke City. Casilla watched that game from the bench, having signed for Leeds 48 hours earlier.

“We have to contribute (to Peacock-Farrell’s development) and take advantage of a time when there are less demanding goals,” Bielsa said. “We'll take advantage of this time to allow him to correct what he needs to correct.

“He’s a very good asset for Leeds. He was formed in the club, he’s very young and he will play as a professional for 15 years at least if he takes advantage of the time positively, either playing or not.

“When you play it limits the chances of seeing the limits your have and the mistakes you make. When you make mistakes that have consequences, it increases the speed of improvement and when you have time without pressure, it's the best time to regain confidence and develop your skills.”

Casilla played in an Under-23s game at Tadcaster Albion on Monday night, a warm-up in chilly conditions for tomorrow’s Yorkshire derby at Rotherham.

Bielsa sat through the match at Tadcaster and got his first sight of the Spaniard in goal. “You can't make an opinion on this kind of game,” he said. “You make an opinion regarding goalkeepers in (first-team) games and we’ll build an opinion on him from Saturday. But what I can tell you is he’s in perfect condition to play.”