Morgan, whose try sealed the Six Nations title with victory over Ireland last March, suffered a dislocated foot during Newport Gwent Dragons' Celtic League defeat against Cardiff Blues two days ago.
He will undergo surgery next week, when the main bone in his foot will be screwed back to the ankle.
The Dragons full-back faces four months out of action, compounding Ruddock's injury worries just five weeks before Wales launch their Six Nations campaign against world champions England at Twickenham.
British & Irish Lions forwards Ryan Jones and Brent Cockbain are already out of the tournament due to injuries, while centre Tom Shanklin is another long-term absentee and Gavin Henson's 10-week suspension after he was cited for elbowing an opponent during a European Cup game, means he misses Six Nations appointments with England, Scotland and Ireland.
Morgan started three of Wales' Six Nations matches last term, scoring two tries in a 46-22 success against Scotland at Murrayfield, then adding another touchdown as Millennium Stadium visitors Ireland were defeated 32-20.
"Kevin will have an operation next week and will be out for four months, which effectively rules him out for the rest of the season," said Dragons manager Jim McCreedy.
"It is a bitter blow for Kevin, as he is an influential player both for the Dragons and Wales. We wish him a speedy recovery."
Morgan missed the 2003 World Cup because of injury, but he recaptured top form last term and would have featured prominently in Ruddock's Six Nations plans.