Northern Ireland face Ukraine at Windsor Park on Saturday, 14 November at 19:45 GMT, before they travel to Budapest to play Hungary in their final match on Tuesday, 17 November.
Both games are live on BBC iPlayer and BBC Sounds.
Manchester United goalkeeper Dermot Mee and Portsmouth forward Eoin Kenny received their first call-ups to the NI squad for the games.
“We’re naturally disappointed to be without Ceadach for the final two games of the campaign. He has been a great asset to the squad in the last international window and everyone here wishes him a swift recovery,” said NI manager O’Neill.
“The wider squad and staff are looking forward to the two games ahead. We’re in a good position in the group and there is plenty to play for.
“The Ukraine game in Belfast will be an important one for us, before we travel to Hungary for another challenging fixture. We’ll be looking to build on the work we’ve done so far and finish the campaign strongly.”
Northern Ireland are the only side left in the entire Nations League who have yet to concede a goal in this year’s competition.
Shea Charles scored a last-gasp winner in Georgia in September before a goalless draw at home to Hungary.
A 3-0 victory over Ukraine was one of the most impressive performances from O’Neill’s young side, and they ended the quadruple-header with another goalless draw, this time at home to Georgia.
If Northern Ireland can top the group they would not only win promotion to League A, but they would also be all but guaranteed of a play-off for Euro 2028.