Schalke are in the middle of a disappointing season and currently sit in 15th place, three points above the relegation play-off spot, despite finishing runners-up to champions Bayern Munich last term.
The club said in a statement: "The door is still open for him to return to the first-team squad and there is no set time frame on how long he will spend with the U23s. The 24-year-old cannot train due to a groin problem at the moment, so will only be doing rehabilitation work for the time being."
Bentaleb joined Schalke permanently from Spurs in the summer of 2017 after spending the 2016-17 season on loan with the Bundesliga outfit.
He left north London in slightly murky circumstance and has since admitted that arguments with Mauricio Pochettino about his first-team opportunities contributed to the demise of his Tottenham career.
Speaking to the Independent in February, Bentaleb said: “I got injured and when I came back, I wanted to play so bad that I had some arguments [with Pochettino]. I was a little bit frustrated, and I was learning basically. Being on the bench is a learning curve.
"I was so hungry to play straight away, and so frustrated when I didn’t play. I know that the coach was just doing this to protect me.”
“I know I made mistakes. But they helped me learn, as a man. And they don’t stop me from sleeping at night. And if I was 24 right now, and I had been at Tottenham, I don’t think I would make the same mistakes.”
But the 24-year-old Bentaleb now appears to have fallen out with the Schalke coaching staff.