England will face Malta and North Macedonia in their Euro 2024 qualifiers next month, but Sterling will play no part in either fixture.
Sterling has struggled for form since his £47.5 million move to Chelsea from Manchester City last summer.
Southgate claimed the 28-year-old has been hampered by a hamstring problem in recent weeks.
When asked about Sterling’s omission, Southgate told reporters: “I spoke with him [Sterling] a week or so ago, a general catch-up.
“He is not happy with his condition physically, he’s been carrying a hamstring problem. He was not really in consideration, so we didn’t get to the point of whether he should be in or out.
“He doesn’t think he is operating at the level he needs, so that was the conversation I had with him.”