Manager Jurgen Klopp fears the 25-year-old may have aggravated the problem he picked up on international duty with Portugal last month on his return to training on Wednesday.
Speaking at the Singapore National Stadium, where the Reds face Crystal Palace in a pre-season friendly on Friday, Klopp told the club’s official website: “Diogo is unlucky.
“Diogo was not involved [against Manchester United] because of an injury he got at the end of his season.
“He trained yesterday fully and got injured again, so that’s really not cool, but we have to wait for the results. He had further assessment this morning, so we have to see.”
Asked if the hamstring was the issue, the German added: “We will see. The same region, yes.”
Meanwhile, Klopp revealed that keeper Alisson Becker is also a doubt.
He said: “Ali was not 100 per cent. He finished the session earlier, was doing the warming up and felt something so now there we have to wait as well. I think pretty much all the others are fine, so that’s it.”