Doctors are hopeful and confident that he will be able to return for the start of the regular season, the sources said.
Doctors are uncertain exactly how or when Baldwin injured the knee. The injury has lingered some, but Baldwin is known for gutting out workouts and he could have aggravated the condition, a source said.
Baldwin has been prescribed rest for the maintenance of the knee before it is re-evaluated later this month. He always has been known to push himself, and if doctors tell him to rest for a week, he's been known to be back in half the time.
On Tuesday, Seahawks coach Pete Carroll described Baldwin's injury as a "sore knee" and called the situation "a little bit of a problem." He said that Baldwin would be sidelined "a couple weeks."
Baldwin, 29, has led the Seahawks in receiving in five of his seven years in the NFL. He narrowly missed out on his third straight 1,000-yard season in 2017 but still made his second career Pro Bowl as an injury replacement.
ESPN's Brady Henderson contributed to this report.