"It appears that he has similar symptoms to those he experienced earlier this season with tendinitis of the patellar tendon," Amaro said. "We will conduct a repeat MRI and will know more about his injury once the MRI is performed [Friday]."
Amaro added that the Phils will make a corresponding roster move on Friday.
Lannan, who is eligible for salary arbitration following the season, missed two months earlier this year due to a strained quadriceps tendon in his left knee, and he said on Wednesday that he feels pain in his knee every time he pushes off that leg. Over 14 starts in his first season with the Phillies, Lannan is 3-6 with a 5.33 ERA.
"It's tough," Lannan said on Wednesday. "I want to help this team out tremendously, go out there every five days. I know when I'm healthy I can go out there and do my job. I feel like I'm letting everybody down -- letting myself down, my family down -- but you have to look at the silver lining. Hopefully, I can get this thing fixed."
Adam Berry is a reporter for MLB.com. Follow him on Twitter at @adamdberry. This story was not subject to the approval of Major League Baseball or its clubs.