So much for that theory.

Fill-in Todd Greene smacked a three-run home run and Posada hit a pinch-hit grand slam, leading the Yankees to a 7-4 win over the Baltimore Orioles Tuesday night at the Stadium.

Posada, who will be unable to catch until at least the weekend, hit for Greene in the eighth inning with the bases loaded. Yankees Manager Joe Torre said that he used Posada in that spot because Greene couldn't remember whether he had ever faced Orioles reliever Mike Trombley, and Torre felt better using the switch-hitting Posada, neutralizing Baltimore's options.

"He's a good hitter, and a switch hitter, which negates what you can do out of the bullpen if you're the other team," said Torre of Posada. "I'm not going to catch him for a while, probably not until a day or two or three after we feel he's ready. We need to have him for the long run."

Posada's slam to right field capped a wild game, which saw the Orioles grab a quick lead as starter Ted Lilly gave up solo home runs to Jerry Hairston and Mike Bordick to lead off the game. Lilly recovered well, though, retiring six of the next seven batters.

"I like the fact that he settled in and didn't let it get the best of him," said Torre of Lilly. "He made some bad pitches on Bordick and Hairston, but he kept us in the game. I thought it was a good outing for him."

"I was wondering what kind of day it was going to be after the first two hitters," said Greene, who caught Lilly a few times at Triple-A Columbus. "My job as a catcher is to try to get a pitcher through that, not let him panic and worry about it, especially a young pitcher like Ted. He didn't, he did a great job."

Down 2-0 in the fifth, Lilly gave up another solo shot to Bordick, his second of the game and sixth of the season. Bordick was 4-for-4 on the night.

"When something like that happens, you're not happy with the result," Lilly said. "At that point, we hadn't lost the game, so you just want to get the next guy out. It's so early, I just needed to focus on getting outs and put that behind me."

Lilly would get a second chance in the sixth, as the Yankees evened the game on one swing by Greene in the bottom of the fifth.

Orioles starter Jose Mercedes was cruising through the first four innings, allowing just one hit. But he ran into trouble in the fifth, giving up back-to-back singles to Paul O'Neill and Scott Brosius to lead things off, bringing Greene to the plate.

Greene, whose contract was purchased from Columbus on Tuesday, crushed the first pitch over the fence in right-center field to tie the game at three. With the 25,134 on hand at Yankee Stadium going wild, Greene's new teammates pushed him out for a curtain call.

"He was completely oblivious. He was putting his stuff on, we had to call his attention to it," said Torre, who added that Greene will start again on Thursday. "He did a hell of a job defensively. He didn't miss a ball all night, and had two good throws."

"I told him to get out there," said Posada. "I was screaming at him, he didn't want to go out. The fans weren't going down until he got up."

"It was very exciting, it was fun," said Greene. "I wasn't going to do anything until someone told me to. I didn't want to seem like I was eating it up. I was getting my shin guards on. To be able to do something like that in my first game here is special."

Baltimore regained the lead in the seventh, as Hairston drove in his second run of the night, singling home Mike Kinkade, who had singled to lead off the inning.

Lilly, who left the game after Kinkade's single, was charged with all four runs on five hits and five walks, striking out three.

Mercedes allowed three runs over 6 2/3 innings, giving up seven hits and three walks. He struck out four in the no-decision.

"I don't think that I pitched all that bad," said Mercedes. "Whenever I give up runs, it's like one inning. After those innings, I don't do all that bad. It just happens to be one inning, after that I'm fine."

"He started getting pitches up," said Orioles Manager Mike Hargrove of Mercedes. "They hit three consecutive first-pitch fastballs for base hits. He lost command of his pitches, he just got the ball up a bit."

With a 4-3 lead, Hargrove called on his bullpen to finish off the Yankees. John Parrish got the O's through the seventh, but Buddy Groom and Mike Trombley couldn't keep New York down in the eighth.

Bernie Williams led off with a single against Groom, and Tino Martinez followed with a double, putting runners at second and third with no outs. After Groom retired Paul O'Neill, Hargrove elected to bring in Trombley to intentionally walk Scott Brosius with Greene on deck.

That's when Torre called for Posada to pinch-hit for his replacement.

Posada fouled off a pitch and took two balls, bringing the count to 2-1. He ripped the fourth pitch he saw from Trombley over the wall in right field, giving New York a 7-4 lead. It was his second grand slam of the season -- and his career.

"I was just trying to hit the ball in the air, try and get the guy at third in to tie the game," said Posada, who took a curtain call of his own following his home run. "That's all I was trying to do."

Mike Stanton, who allowed two hits and no runs in 1 2/3 innings, improved his record to 5-1.

Mariano Rivera pitched the ninth inning, striking out two for his 17th save of the season.

But the postgame buzz was about Greene and his immediate impact on his first day in pinstripes.

"He caught very well," said Torre. "Not only the offensive part, but he did a hell of a job behind the plate, coming in his first day to catch a few different people he hasn't caught before."

"You can't have one second where you don't have full concentration, because you'll make a mistake," Greene said. "That's usually when you notice a catcher -- when they make mistakes. I was hoping not to get noticed.

"It's awesome," he added. "Any time you can contribute in a win, especially in my first game here, my first start -- the way we won that game was very special. I'll never forget that."