Orient skipper Stephen Dawson left the Memorial Stadium on crutches as his team celebrated their first away success in the league 19 attempts thanks to a 3-0 win at Bristol Rovers.
The celebrations in the visiting dressing-room were dampened by concern over the midfielder, who limped off following a lunging 34th-minute challenge from Carl Regan that earned the Rovers full-back a red card.
It was a symptom of the home side's frustration at being two goals down and the tackle was condemned by both managers.
Orient's Russell Slade said: "It was an horrendous challenge. Our captain has heavy bruising down the back of a calf and it's possible there is some bone damage as well.
"We will have him checked as soon as possible. Knowing Paul Trollope, I'm sure he was as disappointed as I was by the tackle. Perhaps it was out of character, but there's no doubt that it was a bad one."
Dean Cox opened the scoring in the 20th minute with a firm finish at the far post. Alex Revell doubled the advantage with a close-range header from Cox's cross seven minutes later and completed the scoring five minutes after the break, firing home from six yards after Paul-Jose Mpoku had got behind the Rovers defence on the left.