Armstrong's wife Carol Armstrong's characterisation of her husband's condition was relayed by another moonwalker, Apollo 17's Gene Cernan, the NBC News reported.
Neil Armstrong, who lives in the Cincinnati area celebrated his 82nd birthday on Sunday, and went to a hospital next day for a stress test.
On Tuesday, surgeons bypassed four blockages in his coronary arteries.
His wife reported that his spirits are high, and the doctors expect no problems with his recovery, Cernan was quoted as saying by the NBC News.
Armstrong became world-famous in 1969 when he and fellow Apollo 11 astronaut Buzz Aldrin landed on the Moon. As the mission commander, it was Armstrong's role to step out of the lander first, descend a ladder and take the first-ever footstep on the lunar surface.
"That's one small step for a man, one giant leap for mankind," he declared.
After Apollo 11, Armstrong worked briefly at the Pentagon's Office of Advanced Research and Technology, then became an engineering professor at the University of Cincinnati in his native state of Ohio.
In a statement, NASA Administrator Charles Bolden said the space agency "wishes Neil Armstrong the very best for a quick recovery from surgery".
"Neil's pioneering spirit will surely serve him well in this challenging time, and the entire NASA family is holding the Armstrong family in our thoughts and prayers. I know countless well-wishers around the world join us in sending get-well wishes to this true American hero," Bolden said.
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