A repair job would have required two spacewalking astronauts to fill the cut with a heat-resistant filler that hardens like caulk, a manoeuvre which has its own risks.
During lift-off on August 8, insulating foam and possibly ice from the shuttle's fuel tank struck its underside, piercing all the way through the heat-resistant tile.
Endeavour docked at the International Space Station on August 10 where astronauts installed a new beam on its structural backbone.
The shuttle is due to return to Earth on Wednesday.
Nasa has been cautious about heat shield damage since the shuttle Columbia exploded shortly before landing in 2003, killing all seven astronauts on board.
The accident was blamed on damage to the wing heat shield caused by flying tank foam during launch that had gone undetected.
Nasa will be retiring the remaining three shuttles in 2010 after construction at the International Space Station is completed.