Aug. 26 Practice Notes from Denver

On one field, safety Antoine Bethea intercepted a Peyton Manning pass, prompting a wild celebration on the 49ers' sideline. At about the same time on the other field, quarterback Colin Kaepernick hit tight end Garrett Celek down the middle for a long touchdown pass.

The 49ers had a difficult adjustment to practicing at a quick tempo at high altitude. And there was a noticeable adjustment period during the 1-hour, 45-minute practice at the Broncos' training facility.

The 49ers' passing game did not make many plays throughout the practice and there were some issues with pass protection, but they finished on a high note with Kaepernick's deep pass to Celek.

Fullback Bruce Miller had an excused absence from Wednesday's joint practices with the Broncos to tend to a personal matter, 49ers coach Jim Tomsula said.

McDonald's injury is not believed to be too serious, as he re-entered Sunday's game briefly after a Dallas Cowboys player initially rolled into his leg. McDonald likely will not play in Saturday's exhibition against Denver.

The 49ers' players said they felt the mile-high thinness in the air as soon as they started going through individual drills. Both the offense and the defense struggled early. Broncos' backup quarterback Brock Osweiller was particularly effective. Meanwhile on the other practice field, the 49ers quarterbacks were struggling to hit their targets downfield or mostly settling for dump-off passes.

On the other side of the complex the 49ers' starting defense had a string of big plays, starting with safety Antoine Bethea's sideline interception of Peyton Manning. Bethea, who spent years with Manning in Indianapolis, said that perhaps Manning thought the 49ers were in a different look.

The 49ers continue to juggle their starting cornerbacks. The top two on Wednesday were Tramaine Brock and Dontae Johnson. On nickel snaps, Brock moved inside and Kenneth Acker played on the outside.