It even goes beyond guys dropping out. Anyone in the top 60 of the World Rankings after this coming weekend makes it in, and the USGA is always conservative in holding spots for any players that might move up. That should open up a few spots, and based upon the strength of the field in Kevin's qualifying sectional, I'd say there is a >50% chance that he tees off at Oakmont.
Just to follow up with some detail on your comment.
As you said, he top 60 in the World Golf Rankings (that's as of next Monday) get spots in the US Open. Last year, the USGA held 6 spots for last minute movement in the rankings. Two of the spots got used, which meant that four went to alternates. If we use number of spots that were available at each of the qualifiers as a proxy for field quality, then the qualifying sites would be ranked as follows.
Walton Heath Golf Club, Surrey, England (13 spots)
Wedgewood Golf & Country Club, Powell OH (13 spots)
Germantown Country Club/Ridgeway Country Club, Memphis TN (10 spots)
Canoe Brook, Summit NJ (6 spots)
Lake Merced Golf Club/The Olympic Club, Daly City, CA (6 spots)
Higashi-Hirono Golf Club, Hirono. Japan (4 spots)
Timuquana Country Club, Jacksonville, FL (4 spots)
Springfield Country Club, Springfield OH (4 spots)
Ansley Golf Club, Roswell GA (3 spots)
Woodmont Country Club, Rockville MD (3 spots)
Lakeside Country Club, Houston TX (3 spots)
Royal Oaks Country Club, Vancouver WA (3 spots)
If this year plays out similar to last (and if my # of spots proxy is valid), then four spots would become available and that last spot would go to Foley (played at Canoe Brook) or a guy from California. Then it would only take one illness/injury dropout to guarantee Foley that last spot.