Strasburg left Saturday's game against the Giants after 3 2-3 scoreless innings because of tightness in his left side. He had an MRI on Sunday.
"It's unfortunate because he felt food and was pitching great," manager Matt Williams said.
Williams said he didn't have a sense of how long Strasburg would be out.
"Not really. We know it's 15, so we'll do what we can in those 15 and see where we're at," he said. "It reacts differently with everybody, and 15 days from now we'll have a better answer. I know that."
Strasburg missed nearly four weeks earlier this season with neck tightness. In three starts since coming off the DL, the 26-year-old righty was 2-0 with a 1.15 ERA.
Overall, Strasburg is 5-5 with a 5.16 ERA in 13 starts.
Williams said he hasn't chosen his starting pitchers beyond Wednesday's game against the Reds. With an off-day Thursday, he could wait until after the All-Star break to pick Strasburg's replacement.
The first 25,000 fans at Sunday night's game received a Strasburg bobblehead commemorating his major league debut against Pittsburgh on June 8, 2010, when he struck out a career-high 14.
Jordan, 26, joins the Nationals for the third time this season. In 16 games with Washington over two-plus seasons, he is 1-8 with a 4.81 ERA.