Johnson, initially diagnosed with an abdominal injury, is expected to be in the hospital for another day for observation, the team said.
The Browns signed Johnson to a one-year deal this offseason, with hopes that he would win the starting nickelback job.
Johnson is just the latest in a series of injuries the Browns have endured during training camp. Center JC Tretter and linebacker Mack Wilson are both out with knee injuries. Running back Nick Chubb had to enter the concussion protocol after taking a hit on the first day of padded practice earlier this week.