Central defender Hummels picked up a calf muscle injury in Borussia Dortmund's German Cup final defeat by Bayern Munich on Saturday.
"He has a muscle fibre tear in his calf and will be out for a few days. At the moment the only thing possible is treatment," Loew told reporters after Germany's first training session in their camp in northern Italy.
Loew, who trained with only 16 players on Wednesday morning, also sounded upbeat about captain Schweinsteiger's recovery with the Manchester United player out of action since March with a knee injury.
"Physically he has done everything asked of him. Sometimes he trained as much as three times a day," Loew said. "He has a good basis now and I am optimistic for the coming weeks."
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Loew said the 31-year-old Schweinsteiger, who has 114 caps, could train fully in terms of running and sprinting but would need a few days before starting to work with a ball.
Germany, drawn in Group C along with Ukraine, Poland and Northern Ireland, are looking for their first continental trophy in 20 years, with the last of three Euro titles back in 1996.
"I am doing well at the moment and I am very optimistic," Schweinsteiger told reporters. "The good thing is that I can deal with these situations. I have had difficult times before."
Schweinsteiger, who had an injury-plagued first season at United and played just 18 league games, also had to race to get fit for the 2014 World Cup with Germany winning the trophy for the fourth time.
"There is a plan for me and I follow it by the hour," he said. "It worked two years ago. Maybe I was even worse back then than today. Little by little I will move forward."
(Reporting by Karolos Grohmann; Editing by Ken Ferris)
