The versatile 30-year-old suffered what the club described as ‘a complex ankle break’ during the reserves’ win against Mansfield Town on Tuesday.
He went down under a challenge on the edge of the penalty area and left the field on a stretcher.
“My initial thought was that I’d felt something break in there,” Sinclair told the club website.
“The swelling came on immediately and the pain was quite significant that night, then obviously the scan on Wednesday revealed the break.
“I came back to England in December of 2017 and from then until the summer I’d been training on my own keeping myself fit so coming in this season it was similar to coming back in from an injury lay-off.
“Getting back on the ball and playing again after the period out has taken a bit longer than I would have liked to get going again and be able to show what I’m about and I feel that over the past month I’ve really been getting near to where I want to be.
“But now I’ll have to recover from this, then start over, and I only feel I’ve shown maybe 60 per cent of what I can give for the club so it’s a massive blow.
“I have broken my left leg previously. On that occasion I was back on the field within three months of suffering the injury so I know what the next few months have in store.
“I just have to remain positive and use this time while I recover to make sure I come back strong.”
The injury comes just as Sinclair was looking to force his way back into Jim Bentley’s plans.
Having become one of the club’s new faces over the summer after a spell abroad, he has only featured on three occasions so far.
His debut came in the 6-0 opening day defeat at Crewe Alexandra before he featured in the Carabao Cup exit at Preston North End and Checkatrade Trophy loss at Carlisle United.
Bentley said: “James has worked extremely hard since being at the football club and has been improving day by day.
“I thought he was excellent in the recent game against Carlisle and was getting close to where we know he can be.
“This is a hammer blow for us and even more so for him; he’s a positive person though so I’m sure he’ll come back stronger.
“We’re waiting on another scan to determine whether an operation is required, but we’ll be looking at at least three months out.”