I don't think it will be a walkover. If his injured guys are still in the lineup then that counts. So he will have still started at least 4 guys, they just will not play. So I am pretty sure you are safe. It doesn't matter if they don't play.
For instance, a player starts five guys and 3 of them foul out your safe. Likewise he is starting 4 guys, only 2 will play. I would be suprised if its a walkover. I think a walkover only occurs when there are not at least 3 guys on the roster for that game.