"I think he's starting to get up there in innings, and in August, arms start to get a little more tired," Gardenhire said. "It looks like he's out there, and he has to force it a little bit more and is working a little bit harder out there. Guys go through dead arms, and maybe he's there right now."
Deduno's velocity, however, has held steady, as his fastball averaged 90.4-mph on Tuesday, which is in line with his average fastball velocity of 90.2-mph this season, according to FanGraphs.com.
But Gardenhire said he's noticed a decline in Deduno's command, as he's had trouble locating his fastball in recent starts.
"[Tuesday], he was bouncing balls and threw some fastballs way out of the zone," Gardenhire said. "So it looks to me [like] he's a little dead-armed or a little tired. But if you ask him, he'll never admit it."
Rhett Bollinger is a reporter for MLB.com. Read his blog, Bollinger Beat, and follow him on Twitter @RhettBollinger. This story was not subject to the approval of Major League Baseball or its clubs.