Loew counts cost of Germany's six-goal romp
Germany's midfielder Marco Reus reacts on the pitch during a friendly football match against Armenia in preparation for the FIFA World Cup 2014 on June 6, 2014 in Mainz, central Germany - by Daniel Roland
The 25-year-old Borussia Dortmund winger was rushed to hospital after twisting his left ankle in a challenge just before half-time.
A scan revealed a partially torn the ligament in his left ankle, which threatens his Brazil campaign.
The German Football Association (DFB) have said a decision will be made later Saturday before the team fly to their World Cup base Porto Seguro from Frankfurt at 22:00 local time.
"Marco has been in great form and a bad injury would be very bitter," admitted captain Philipp Lahm before the diagnosis.
The injury put a dampener on an impressive display as goals by Lukas Podolski, Benedikt Hoewedes and Mario Goetze, who scored twice, floored minnows Armenia, who had a penalty converted by Henrikh Mkhitaryan.
Replacement Miroslav Klose, who turns 36 on Monday, made history when he headed home their fourth to become Germany's all-time top scorer.
Klose netted his 69th goal for Germany on his 132nd appearance to finally break Gerd Mueller's previous mark of 68 goals in 62 matches for West Germany.
Loew had said he wanted a confidence boosting win and his side upped the pace with five goals in 17 second-half minutes.
"Our pre-World Cup training camp was intense for all the players and you could see they were fresher today," said Loew.
"We struggled to take our chances in the first half, because the opponent stood very deep."
The Germany boss said the Brazilian climate will be a factor when his side kick-off their Group G campaign against Portugal on June 16, but is something they are planning for.
"There are lots of things which need to work at a big tournament," said Loew.
"When you come from Europe (to Brazil), you can't assume to be able to play the same type of football, which we can here.
"I have experienced the temperatures and humidity (in Brazil), you can't expect to go flat out for 90 minutes.
"So efficiency in taking your chances is an important topic for us."
Despite Reus' injury, there were several plus points for Loew.
Both captain Philipp Lahm and his deputy Bastian Schweinsteiger played after enforced injury breaks.
Arsenal star Lukas Podolski provided two assists and a goal in an impressive second-half display for Reus on the left wing.
"If you look at the statistics, we had lots of goal chances and we need knock them in earlier," said the 29-year-old Podolski, who scored his 47th goal on his 114th appearance.
"We showed what we can do in the second-half, now we have to prepare well for Portugal, who are a tough team and will be a different calibre than today.
"It's very important to win your first game."