Hernandez came off after just three minutes of Saturday’s 2-1 Championship reverse at Fulham, with Bielsa confirming that the influential Spanish play-maker will miss the club’s festive programme of fixtures and most of the January schedule.
Dallas will operate in midfield in the home game against fifth-placed Preston on Boxing Day, with Gjanni Alioski, who came on for Hernandez at Craven Cottage, starting at left-back.
On the importance of Dallas, a real unsung hero for Leeds so far this season, Bielsa said: “I am going to play Alioski (at left-back) and Dallas in midfield.
“He (Dallas) is a dynamic player and is flexible.
“He can play on the left or the right side with no problems and has good mobility. He can appear in the last part of the pitch and has a good tempo.
“He can shoot and finish as well and also cross from the sides. He defends and attacks.”
Bielsa also confirmed that Tyler Roberts and Jamie Shackleton are available for Boxing Day, but suggested they require further time in training to boost their sharpness and fitness levels before coming into his thoughts to start.
Left-back Barry Douglas has been suffering with sickness, the Argentine added.
Bielsa, who said that there were no updates regarding loan striker Eddie Nketiah, with his parent club Arsenal to review his situation in the New Year, commented: “Tyler and Shackleton are available. Douglas is sick. We still don’t know when (Adam) Forshaw will come back to the group.
“They (Roberts and Shackleton) need time before they are competing.”
Meanwhile, visitors Preston have also been handed a significant injury blow in midfield themselves with key schemer Daniel Johnson remaining sidelined.
Preston manager Alex Neil said: “DJ is a big blow for us.
“What you need with Leeds is three things – you need tactical awareness, you need legs and the ability to take the ball, handle it under pressure and shift it, because they will go man for man all over the pitch.
“You need to be able to do all three things and DJ is one of the players who gives us all of that, so it is a big, big blow that he is missing this one for us.”
Second-placed Leeds hold a nine-point buffer over Preston, with Bielsa’s side trailing leaders West Brom – who they visit on New Year’s Day – by three points, while boasting a healthy eight-point advantage over Sheffield Wednesday, who occupy third place at Christmas.
United secured a comprehensive league double against North End last term, triumphing 3-1 in the league fixture at Elland Road in September 2018, while a brace from Patrick Bamford saw Leeds win 2-0 at Deepdale in April.
Neil did have the consolation of being the first manager to inflict a defeat upon Bielsa’s Leeds when Preston won 2-0 in a Carabao Cup tie early on in the 2018-19 campaign, but the Scot admits he is under no illusions about the degree of difficulty against a Whites side who are unbeaten on home soil since being beaten 1-0 by Swansea City at the end of August.
Only Preston have taken more Championship points in front of their own supporters so far this season than Leeds, with the Red Rose side having triumphed just twice in the league on their travels in 2019-20 by contrast.
Acknowledging the extent of the task facing his side, Neil added: “The last time we were there last year, we were poor and we really struggled – they beat us comfortably and it is one of the most difficult venues in the league to go to.
“The fans really get behind them, the atmosphere is excellent and their team is a really good team.”