However, his recovery has been proceeding smoothly and he could be ready to work out very soon.
Lascelles, speaking to the club matchday programme, said: "For the first 20 seconds after going down, I felt pain but after that it more or less went and I thought maybe it was just a bone on bone impact injury. I got up had a little jog and it didn't feel too bad.
"I didn't feel it pop or anything like that - but when I went back on I knew something wasn't right. I genuinely didn't expect it to be anything like an ACL though until the scene confirmed the full extent of the injury."
He then added: "The operation went really well and I am up and about, weight-bearing and looking to get off the crutches and start work in the gym soon.
Sadly I can't help the team on the pitch any more this season but I'll be doing everything I can to help off it. The boys know what is at stake."