Archer saw a specialist, Dr. William Meyers, on Tuesday morning in Philadelphia and re-joined the team in Washington D.C., before Tuesday night's game against the Nationals.
"Considering all the possibilities, I think this was the best-case scenario outside of being healthy," Archer said. "We don't have an exact date for how long I'm going to be out, but it's not going to be too long. According to what the expert says."
More dire results could have been an oblique strain or core-muscle repair surgery.
"But it was not even close to anything like that," Archer said. "Very minor. The main thing is right now, I need rest. I'm going to do that for the next few days, and just play it by ear.
"... It's a strain, so there is inflammation and just wear and tear. I've been doing it for a long time without stopping. So, I think my body's telling me, 'Maybe you need to slow down for seven to 10 days.' And then hopefully pick right back up where I left off."
Archer said it's hard to know how much action he'll miss, but "from what [the doctor] saw, he said seven to 10 days. I don't know if the Rays want to push it like that. I don't know if I would throw a large BP, or have a start in the Minors. He said seven to 10 days, so it's minor."
Archer's next scheduled start would have been Friday at Tropicana Field against the Mariners. Rays manager Kevin Cash said Matt Andriese will fill the gap that day, whether that means starting the game, or entering the game after the "opener" to pitch the bulk of the innings after the first or second innings.
Castillo could be used in late-inning situations, according to Cash. When asked if Castillo might be asked to get the last three outs of the game, the Rays' manager suggested the right-hander might be asked to get the final six outs.
• Cash confirmed on Tuesday that left-hander Anthony Banda would have Tommy John surgery. On Monday, the Rays would not confirm a report by the Tampa Bay Times about Banda needing the surgery.
• Pitching coach Kyle Snyder returned home to St. Petersburg on Monday after being hospitalized in Seattle over the weekend with an unspecified illness.
• Class A Advanced Charlotte first baseman Nate Lowe has been named the Florida State League Player of the Month for May. He hit .356/.442/.611 in 24 games with five home runs and 19 RBIs. Lowe, 22, was also the FSL Player of the Month for April.