The former Ipswich forward hobbled off 15 minutes from the end of Saturday's 4-1 defeat to Dundee United after pulling up with a hamstring problem.
He underwent a scan on Tuesday and Hearts boss Gary Locke's worst fears were confirmed on Wednesday when it emerged that the talisman has a tear on his hamstring, which means another spell on the sidelines.
The 29-year-old has already missed two months of action with a knee injury and his lay-off leaves Locke with just two other players older than the administration-hit club's squad average of 22.
The manager told Hearts' official website: "It's a massive blow for us and for Ryan. He pulled up during the Dundee United match last Saturday so we decided to send him for a scan to get to the bottom of it. We had a niggling feeling that it was something serious and that was confirmed when the results came back.
"There's a tear on his hamstring so he'll miss the next four to six weeks, which is less than ideal. It's a shame for Ryan as he's not long returned from a long spell out.
"One good thing though is that the lad is a quick healer, so hopefully he gets a bit of rest and then we can manage his return."