yes, it will. i suspect if you have 6 guys foul out (forcing a walkon to come on), the walkon coming in will not be the straw that broke the camels back, so to speak - you are going to lose regardless. but yeah, it would definitely happen.
the walkon could probably come in earlier than 6 guys fouling out, too. in extreme fatigue cases, players can come in, but, its got to be pretty extreme - so it sort of depends where you are at in the game, if you have 4-5 guys foul out, if you might have a walkon show up.