Hammel's next regular start would have been Thursday, but that date was pushed back to Monday or Tuesday to give him time to recover from right knee soreness that has been slowly worsening since it first cropped up two or three starts ago.
Hammel said Wednesday that an MRI revealed fluid and inflammation, but the knee had been improving with treatment.
"He could have pitched today if we had to," Showalter said. "If it was a different part of the season, he probably would have pitched, but we want to be real cautious."
Hammel dealt with a similar injury to his left knee, which was operated on after the 2008 season, so Showalter is trusting him to appropriately handle his latest ailment.
"He's got his arms around what he's dealing with and he feels like if he can get a couple extra days, or three or four, he should be able to manage it the rest of the season," Showalter said.
Missing Hammel for any stretch of time would be a significant blow, as he has been far and away Baltimore's best starter this season. Through six starts, the righty is 4-1 with a 2.09 ERA. In his most recent outing on Saturday against Boston, Hammel threw 6 2/3 innings, allowing two runs on four hits.
Although Eveland had been projected as the starter for Friday's game against Tampa Bay for some time, Showalter confirmed those reports Thursday. Eveland was pulled from his Monday start with Triple-A Norfolk after five scoreless innings and 63 pitches.
Eveland, who made the trip to Baltimore on Thursday, has been Norfolk's most effective starter this season, posting a 3-2 record with a 2.21 ERA. The lefty has not allowed more than three runs in any outing this season.
"He had a good year in a very hitter-friendly park in Triple-A and has had some success at times in the big leagues," Showalter said. "He's pitching real well down there, pitching as well as any of their starters are, so I think we're lucky to have him. We'll see if Tampa agrees with me."