McEnroe's name was among the four he announced Wednesday for the best-of-five series at Santander, Spain, July 21-23. The 41-year-old U.S. captain said he'd limit himself to doubles, "no question. You can bet your life savings."
Agassi strained his back in a minor car accident this weekend, while Sampras was hobbled by a shin injured while winning Wimbledon.
"We've lost two of the best players that ever played. This certainly has not turned out the way I had it planned out in my mind," McEnroe said, putting his team's chances of beating Spain at 10 to 20 percent.
Agassi and Sampras are ranked third and fourth in the ATP Champions race.
McEnroe can substitute any of his four players up to 10 minutes before the July 20 draw for the semifinal, which will be played on slow, red clay. And the captain said he will talk to other players, chiefly Paul Goldstein and Vince Spadea (who ended an ATP Tour-record 21-match losing streak by winning in the first round at Wimbledon).
"I haven't actually decided to compete at the moment. I've been looking at calling another couple players I haven't been able to reach," McEnroe said. "These pullouts came very recently. This is a stopgap solution at the moment."
"There's a chance I won't play. There's also a chance I will."
Agassi was injured Sunday riding from the airport to his Las Vegas home, the U.S. Tennis Association said, and he can't raise his right arm above his shoulder.
"This condition is terribly disappointing to me as my teammates and I have all worked very hard together to represent our country in the Davis Cup competition," Agassi said in a statement.
McEnroe sounded more forgiving of Agassi - whom the captain called "my main guy" than Sampras. The 13-time Grand Slam champion said Tuesday he probably was too hurt to play Davis Cup but hadn't talked with McEnroe at the time.
"I don't know how injured (Sampras) is. There's no way o me knowing that," McEnroe said. "Davis Cup was something that was a priority to me. But, who am I to say? I wish he had made an effort to do it, and perhaps if he played a doubles match and been there, that might have been inspiring. ...
"Who knows, maybe he'd be risking further injury if he played right now."
McEnroe was passionate about this international event during his playing days.
The Hall of Famer helped win five Davis Cup championships and had a 59-10 record in 12 years of competition. He holds six U.S. records, including most singles (41) and total victories.
He became captain in September, replacing Tom Gullikson, and has talked of putting himself on the team.
"At the moment, there's not a lot of guys out there who have gotten themselves into a position where they should be playing (Davis Cup)," McEnroe said. "We'll see what I do. I could lose 6-1, 6-1, 6-2 and that would be embarrassing. If you go out there and don't play well, everyone can take potshots at you. It's not like I'm playing a couple of stiffs."
The United States, which last won the championship in 1995, had to rebound from 2-1 deficits in each of the first two rounds this year against Zimbabwe and the Czech Republic.
Now the Americans face a tougher opponent in Spain, which has three team members - Alex Corretja, Juan Carlos Ferrero and Albert Costa - who reached the quarterfinals at the French Open, the only Grand Slam event played on clay.
"We had our hands full even if we had the best guys. They have three of top 10 in the world on clay," McEnroe said. "We're going to be huge underdogs. If we can pull this one out of the hat it will be a real victory."
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