Hamels went on to say he's not concerned about the inflammation.
"We've got five pitchers who I think have inflammation in their shoulders right now," he said. "Because it was uncomfortable in the last start, maybe there was more inflammation. It's just a matter of doing what could be considered a rehab and strengthening up everything and trying to flush out the inflammation as fast as possible."
Hamels was optimistic he'd miss only one start.
"I'm throwing [Wednesday] and I've got a bullpen [session] in two days, and then whenever my spot comes up," he said.
Roy Oswalt will start Friday at Washington. He'll be on normal rest because of Sunday's postponement and Monday's off day. Kyle Kendrick will start Saturday against the Nationals.
Phillies general manager Ruben Amaro Jr. said the inflammation is the result of Hamels' workload and not from an injury.
"It didn't come from one pitch," Amaro said. He expressed relief the MRI showed no structural damage.
"We're pretty pleased," he said. "He's got some inflammation in his posterior rotator cuff, but no major changes. We just want to be cautious about it. We've got a long way to go until the end of the season and so we want to be careful with him."
Hamels was pulled after five innings of the 4-2 loss to the Nationals. He lacked his typical command and his fastball didn't have its usual velocity. It was only the fifth time in 25 starts Hamels didn't pitch into the sixth inning.
Afterward, Hamels surmised the stiffness was the result of the normal fatigue many pitchers endure at some point during a season. He has pitched 172 innings, which was tied for third in the National League. Including playoffs, he pitched 223 innings last season.
Contact staff writer Ray Parrillo at email@example.com or @rayparrilloinq on Twitter.