Leeds close on West Brom
Victory left second-placed Leeds just a point behind the Baggies, who suffered a surprise setback later Saturday when losing 1-0 at home to relegation-threatened Wigan in a match where Sam Morsey's 73rd-minute goal at The Hawthorns moved the Latics out of the bottom three.
Third-placed Fulham remained in touch with the leading duo thanks to a 2-0 win over Preston, with David Nugent's 58th-minute own-goal the difference until Aboubakar Kamara scored in stoppage-time to the relief of the home crowd at Craven Cottage.
Only the top two clubs at the end of the regular season are guaranteed automatic promotion to the Premier League with the next four sides going into the play-offs.
The eve of Saturday's game had seen Casilla hit with an eight-match suspension after he was found guilty of using racist language towards Charlton's Jonathan Leko during a Championship match on September 28.
Fallen English giants Leeds, however, did just fine without him at Hull's KCOM Stadium on Saturday.
Stand-in goalkeeper Illan Meslier had little to do, while goals from Luke Ayling and Pablo Hernandez, plus substitute Tyler Roberts' late double, made it four wins in a row for a Leeds side managed by veteran Argentinian boss Marcelo Bielsa.
"He was calm and confident and he took risks," said Bielsa of Meslier. "He gave us the feeling that he's confident."
As for Casilla's ban, he added: "We must accept the rules of the competition and accept those decisions.
"All of us are against any kind of discrimination, this is very clear. All of us are against the racists, he (Casilla) is the first one.
"From us he will feel all the love he feels from us."
Defeat extended Hull's poor run of form to 10 Championship games without a win and left them firmly mired in a battle to avoid relegation to English football's third tier.
Luton salvaged a 1-1 draw with fellow strugglers Stoke thanks to James Collins' stoppage-time penalty, after Sam Vokes had given the visitors a ninth-minute lead at Kenilworth Road.
Blackburn Rovers' Bradley Johnson also scored in added time to secure a 2-2 draw with Swansea.
Sam Gallagher had put the hosts ahead at Ewood Park before goals either side of half time fro on-loan Liverpool striker Rhian Brewster (45) and Andre Ayew put the Welsh side into the lead.
Freddie Woodman saved a penalty from denied former Swans striker Danny Graham before Johnson's late effort levelled the scores.
Barnsley were left above bottom-placed Luton on goal difference alone after a 2-0 loss at Reading, who moved eight points clear of the relegation zone thanks to goals from Yakou Meite and George Puscas.