The left knee injury was suffered during the Chargers' organized team activities Tuesday morning.
With Ingram presumably lost for the season, San Diego is bringing in free agent Dwight Freeney on Wednesday for a visit, a league source told ESPN NFL Insider Adam Schefter.
San Diego's new general manager, Tom Telesco, spent the past 15 seasons in Indianapolis, where the Colts drafted Freeney.
Ingram was San Diego's first-round pick in the 2012 draft. He had 41 tackles (21 solo), one forced fumble and one sack for a young, developing linebacking corps.
Ingram thanked Chargers fans for their support in a tweet Wednesday afternoon, writing: "Thanking everyone who said a prayer for me!!! Minor set back for a major come back!!"
ESPN.com's Bill Williamson and ESPN NFL Insider Adam Schefter contributed to this report.