Querrey was chosen ahead of Blake for the best-of-five series against Spain on outdoor clay Sept. 19-21 in Madrid.
"James is just exhausted physically and mentally after a grueling summer," McEnroe said. "He said he needed a break and we respect that. This is also a great opportunity for Sam, who has had a solid year, strong results on clay and has been a loyal practice partner for this team."
Last year, Blake teamed with Roddick and the Bryan brothers to lead the Americans to their record 32nd Davis Cup title.
Querrey was listed on the official nomination released by the International Tennis Federation, but it noted it was still awaiting confirmation on his eligibility. Querrey, ranked No. 40, has never played in the Davis Cup.
Last week, McEnroe said Blake would be on the team with Roddick and the Bryan brothers for a record 11th straight time.
Nadal, who lost to Andy Murray in the semifinals of the U.S. Open on Sunday, may get a few days rest before joining the team.
"He lost a match, but the tiredness and stress are within the usual limits," Sanchez Vicario said of Nadal, who may not attend practice Saturday. "He is very enthusiastic about this tie."
Argentina will host Russia on outdoor clay in Buenos Aires in the other semifinal series.
Federer rebounded from losses in the French Open and Wimbledon finals to win his 13th Grand Slam title on Monday at the U.S. Open -- one shy of Pete Sampras' all-time record.
Federer will play against visiting Belgium on indoor hard courts in Lausanne, and Murray will help Britain against Austria on the grass at Wimbledon.
The Davis Cup final will be played Nov. 21-23.