"This is an MRI of my hamstring, The white stuff surrounding the muscle is known in the medical world as anti-awesomeness," he wrote, providing a link to a picture of the MRI image.
Dr. Michael Kaplan, ESPN's sports medicine expert, provided his own analysis of Foster's injury Wednesday and said that it is significant, based on the image the running back posted.
"The MRI scan tweeted by Foster demonstrates considerable muscle damage with bleeding and swelling," Kaplan said. "Treatment is conservative. Rest, physical therapy with stretching and ultrasound, and anti-inflammatory medicine will be employed. Foster will use a gradual strengthening program when he's ready. A return to the field would not be expected before three to four weeks. Premature exertion risks re-tear and longer convalescence."
The NFL said in a statement to ESPN NFL Insider Adam Schefter that the league's Twitter policy did not cover Foster's actions and any discipline would be at the team's discretion.
"We are aware that Arian tweeted a picture of his MRI," Texans general manager Rick Smith said in a statement. "It is a violation of team policy to disclose medical information via social media and he is aware of that. We will handle the situation internally."
Foster defended providing the image to his Twitter followers with a tweet later Wednesday.
"If I had a 'significant injury' why post it? I'll be fine, it was jus meant to make fun of the whole situation. Humor is lost nowadays," he wrote.
On Sunday, Texans coach Gary Kubiak said he was optimistic that Foster would be ready for the team's regular-season opener against the Indianapolis Colts.
Foster sat out Houston's preseason opener, then rushed five times for 47 yards and two touchdowns in the Texans' 27-14 win over New Orleans on Aug. 20. He was hurt on his eighth carry against the 49ers on Saturday.
"Looks like we're dealing with the same thing we were dealing with at the start of camp," Kubiak said Sunday. "Hopefully, we can get him back on the rehab train, and get him ready for opening day. He'll be day to day."
Kubiak said head athletic trainer Geoff Kaplan offered a "very positive" outlook for Foster's recovery, though Kubiak stopped short of saying Foster would definitely be healed in time for the Sept. 11 opener against Indianapolis.
"Until he hops back out there, there's a concern," Kubiak said. "But it sounds like we're heading in the right direction."
Information from ESPN.com senior NFL writer John Clayton and The Associated Press was used in this report.