According to an NFL Network report, McDowell was involved in an ATV accident and suffered a concussion, along with facial injuries.
McDowell said in a statement posted on Twitter that his injuries aren't life-threatening and won't put his career in jeopardy.
"Unfortunately, Malik McDowell was involved in a vehicular accident, in which he suffered an injury during the NFL break period," the team said in a statement. "As a precautionary measure, Malik has remained back in Michigan under the care of physicians there. Our medical people have been in constant communication with his physicians, and they have been monitoring the situation. At this point, it is important for Malik to stay at home and rest. We consider this a long-term relationship and will do whatever is in the best interest of Malik. We look forward to Malik's return to Seattle and will update you with any changes. We placed him on reserve/did not report today."
Seahawks coach Pete Carroll was asked if there's a possibility that McDowell will be out for the year.
"We'll see. I don't know that," Carroll said.
"It's challenging. He had extraordinarily high hopes to be here and be with us, and he's not able to. So everything's above board and cleared -- what we're doing. But it's going to take a little while just because this is the first time there's been a statement to come out, and we're just honoring the family in how to handle this properly."
The Seahawks drafted McDowell out of Michigan State with the 35th overall pick. The team had plans for him to contribute as an interior pass-rusher as a rookie, and Carroll raved about McDowell's progress in the spring.
Asked if it's a long-term injury, Carroll said, "We'll wait and see. We'll see what's going on. It's been a little while since the accident happened, but we're still waiting to figure all that out."