Escobar said he heard something pop while running out a grounder in the seventh inning against right-hander Todd Williams.
Escobar, who was called up on Saturday, said he didn't say anything to anybody on Sunday because the hamstring was tight and felt it was nothing serious. But the pain became worse on Monday.
Manager Frank Robinson acknowledged that he was miffed that he wasn't informed right away.
"I was surprised to hear it, period. If he hurt it yesterday, I would expect him to let us know yesterday and then check on him today," Robinson said. "I'm not looking at it as anything other than he is missing an opportunity
and that's it. We can use him."
Escobar's career has been riddled with injuries. He was the Mets' No. 1 prospect in 2001, but after he was traded to the Indians in a deal for Roberto Alomar, he found himself on the disabled list in three out of the next four years because of knee and foot problems.
General manger Jim Bowden acquired Escobar from the White Sox in February 2005 for outfielder Jerry Owens. Escobar didn't play a game for the Nationals that year because of a right foot fracture. In fact, he was still having problems with the foot during the exhibition season and didn't start
the Minor League season on time.
"It's frustrating. It was my second game. I'm trying not to think about. I'm just trying to go with the flow," Escobar said.
In the meantime, Damian Jackson replaced Escobar in center field for Monday game.
As for Day, he left Monday's game against the Astros because of right shoulder soreness. He is on the disabled list for the fourth consecutive season, and it's the third time in four seasons in which he has been placed the DL because of shoulder problems.
After Day gave up a double to Willy Taveras in the fourth inning with two outs, pitching coach Randy St. Claire came to the mound to check on Day. Suddenly, manager Frank Robinson and head athletic trainer Tim Abraham
jogged to the mound to find out what was wrong. A few minutes later, Day left with game with Abraham at his side.
It was revealed that Day had right shoulder tendinitis, but he will have an MRI taken on Wednesday to determine how serious the injury is.
Day, who felt tightness in the shoulder, said he knew something was wrong when he hit Eric Munson with a pitch in the third inning and walked pitcher Wandy Rodriguez in the fourth.
"It's really discouraging. It's one of those things where it has been so up and down the last couple of years. I just want to find out what the matter is and why this keeps coming back," Day said.
The Nationals claimed Day off waivers on April 26. That day, Dr. Tim Kremchek, the team's medical consultant, revealed that Day had a muscle spasm in the back of the shoulder. Kremchek had recommended that Day go on the disabled list, but Day said he felt fine and he went on to start five
games for the Nationals.
The Nationals don't need a starter in Day's spot until Saturday afternoon, when they play the Dodgers. Bowden indicated the Shawn Hill, who was recently promoted to Triple-A, could get the nod or Bowden may make a trade.
Hill was 3-2 with a 2.64 ERA for Double-A Harrisburg, and he last pitched for New Orleans on Sunday, giving up two runs in five innings.
By adding Rivera and Ramirez, the Nationals have 12 pitchers on the 25-man roster. Rivera, 28, is 1-1 with a 1.59 ERA in 12 appearances for New Orleans. This is his second stint with the Nationals. He was recalled on April 14, but never appeared in a game. He was optioned back to New Orleans
five days later.
Ramirez, 27, was 2-1 with seven saves and a 0.99 ERA in 19 appearances with New Orleans upon earning his first Major League promotion.
"We need to go to 12 pitchers again and we need to help this bullpen out," Bowden said. "Ramirez has been the best pitcher in Triple-A. Both [pitching coach] Steve McCatty and [manager] Tim Foli recommended him. Ramirez has been dominating down there. He deserves the opportunity. Saul Rivera has been solid all year down there and was solid in Spring Training. Both of them should give us innings and gave some relief to our bullpen, which has been overworked."
Drese has been on the DL since mid-April because of elbow problems.
Injury report: John Patterson, on the disabled list because of a right forearm strain, had the day off on Monday and will play catch on Tuesday. He will be reevaluated on Wednesday to determine when he will have a bullpen session.
"He was a little bit stiff and sore, but that has come and gone. He felt better today and we'll see how it is tomorrow. That's why we are not planning anything long term for him," Abraham said.
One of the reasons the Nationals didn't add an outfielder on the roster on Monday is because Jose Guillen, out for the last five games because of a right hamstring strain, said he is ready to play on Tuesday against Astros. He went through agility and running drills on Monday afternoon and didn't
feel any pain.
Guillen will go through the same drills on Tuesday afternoon and is expected to play that night.
Send in your questions: The Nationals mailbag will run on Tuesday. Send in your questions now.
Stat of the day: Nick Johnson has hit the most home runs at RFK in a Nationals uniform. He has hit 10 home runs in 249 at-bats entering Monday's action.
Did you know? The Nationals have made at least one error in a season high seven straight games. In fact, they have made 21 errors during the month of May, which leads the National League.
FYI: Thom Brennaman and Steve Lyons will do the play-by-play between the Nationals and Dodgers on Saturday for FOX.
Coming up: The Nationals play the second game of a four-game series against Astros on Tuesday night at RFK. Right-hander Ramon Ortiz will get the start for Washington, while right-hander Fernando Nieve gets the nod for Houston.