As noted, United States Champion John Cena underwent Surgery back on July 28th to help him breath better after breaking his nose in a match with Seth Rollins on RAW.
WWE has since then removed him from all WWE Live Events, RAW, and SmackDown tapings. Cena is slated to return to RAW television taping on August 17th to continue his feud with Rollins. Even though his nose was a mess, he was back in the gym this week.
Source: Wrestling Observer Newsletter