I'm not too concerned about Alex coming back.
Harbaugh has always said that QB spot will be an open competition and will be earned. Even yesterday, he said during the Kaep's press conference. Check out the video at 6:15
Alex couldn't beat Rattay in 05. He couldn't beat out O'Sullivan in 08 or Hill in 09. So its pretty much a guarantee Kaepernick will start week 1.
If Harbaugh's plan was too find a veteran who could bridge the gap or buy some time until Kaepernick was ready, he'll soon find out that his veteran of choice will struggle to learn his system which everyone knows by now.