Honestly don’t think Alguersuari will join Sauber. He already announced he had a seat for next year, way before Hamilton made his decission to join Mercedes, and thus Sauber didn’t knew they’d loose Perez.
I rather think Alguersuari will join Caterham and replace Petrov/Kovalainen.
As for Sauber, they probably stick with Kobayashi for continuality, and take in Kovalainen from Caterham.