McCallum, a summer signing following his release from Premier League side West Ham United, picked up the injury in last Saturday’s 1-1 draw at Northampton Town having come off the bench in the second half, and has been ruled out for at least six months.
“I’d like to thank the fans for their messages of support over the last few days, it means a lot to me,” said McCallum.
“I have received plenty of messages over social media. It was disappointing to find out the news of the injury, but I am already fully focused on my recovery and I can’t wait to get back and pay all the fans back for the support they have shown.
“I was at the Carlisle United game on Tuesday and I wanted to get out on the pitch and help the boys out.”
The striker had hit the ground running with three goals in the opening weeks of the season as they O’s set the early pace at the top of the League Two table.
“I felt I was really staking a claim for first team football and making a mark at the club, but that’s football and injuries do happen,” he added. “I felt a sharp pain in my knee straight away but when Pete (Webb) came on and worked on it the pain eased off. I then went to get up but as soon as I put weight on it gave way.”
The 22-year-old spent a similar amount of time on the sidelines after suffering a stress fracture of the spine during 201415 pre-season. With that experience, he believes he is mentally prepared for this lay-off.
“I was out for a similar amount of time with the back injury so I know how to mentally deal with spending time on the sidelines,” he said. “It will be tough but I will be cheering the boys on every week to make sure we can try and build on this.”