Dallas suffered a femoral fracture when he attempted to make a challenge on Manchester City's Jack Grealish in the first half of last weekend's Elland Road game. It was immediately obvious that the 31-year-old had done himself serious harm as he pounded the turf in agony - Dallas is not a man prone to theatrics.
A stretcher was quickly produced and the Northern Ireland international was taken to Leeds General Infirmary before being transferred to London for surgery on Sunday.
Leeds initially released a statement revealing that Dallas would be out of action for a 'lengthy period' and Marsch admits it will be at least six months before he can play again.
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"He had a successful surgery," said the American.
"It was a long surgery five and a half hours but they believe that the outcome was very positive and and you know they say it'll be a long recovery, but they believe that he will have a full recovery.
"He has had something like 400 500 messages from fans, family and friends. We all know that he's loved and I think that this moment proved that more than anything you know, I don't want to speak for Stuart but you know, I've heard he's been very appreciative of so many people reaching out and extending their concerns and their love toward him. So we all hope the absolute best for Stuart and we hope that he is soon pain free and back here in the training centre so that we can really start his recovery.
"I think we can safely say it'll be six months, but it could be longer and I don't think anyone wants to rush him. I think the bone has to heal. And then they have to look at all the things like the articular cartilage and the meniscus, in which everything seems to be relatively intact. The ACL was intact. You know, so I'm not a doctor but a lot of the main structure of the knee is in place and then it's just got to heal a little bit overtime and see how it reacts to more and more.
"He's such a big part of the group. He's such a fantastic person and player. I think he had the current ongoing record for most consecutive matches. So he's also an Iron Man and it's a it's motivation for us. But more importantly, I think it's an indicator for the fact that Stuart will be back and stronger than ever."
Dallas posted on social media earlier in the week expressing gratitude for the support he has received since suffering the serious injury.
"Huge thank you for all of the messages I’ve received in the past few days," he said.
"I’ve had amazing support from across the world of football - but particularly from Leeds and Northern Ireland. I’d also like to thank Dr Williams, Dr Sarraf and the medical team at Leeds United. The support I’ve received has meant the world to me and my family. Now it’s time to focus on my rehab, I will be back from this stronger than ever. In the meantime I’ll be supporting my teams every step of the way. #MOT #GAWA"
His loss is a huge blow for the squad and the club's survival hopes. Dallas has started each and every Premier League game since their arrival in the top flight in 2020 and has filled several roles thanks to his ability to operate comfortably in different areas of the pitch. Stints on both sides of the defence and in midfield helped him to take the Player of the Year award last season, having initially joined the club as a winger in 2015.
Dallas has made 267 appearances in all for the Whites since his move from Brentford and played a key role in the club's promotion season under Marcelo Bielsa, who the player has often credited for transforming him on the pitch.