Carpenter/ Reyes question

I saw reports that said Carpenter may go on the DL if he doesn't make his next start.  There are also reports that if Carpenter cannot make his start Anthony Reyes will get the call.  But I thought Reyes was optioned back to Memphis after his start in KC.  Isn't there a rule that if a player is optioned to the minors he must remain there for 10 days?

If this is the case, why would the Cards option Reyes back to Memphis knowing that Carpenter just missed his start?  It does not make sense to me.  If this is the case they should have went ahead and optioned Duncan first and then seen how Carpenter.  Duncan was optioned when Ponson got his start today.