Ruud underwent a scan on Friday to determine the extent of the damage to his calf with the test showing that the injury had now affected his Achilles requiring up to six weeks of recuperation.
"It's not great news for us at all," Ferguson told Sky. "What started off as a calf injury is now an Achilles injury. The problem is what to do next. The recommended treatment is therapy and doing a complete rehab on his injury. Hopefully, it's four weeks or six weeks at the very most, but we don't know for certain."