Mariano Rivera remains scheduled to take the mound on Tuesday for a bullpen session, and the Yankees are hopeful that their All-Star closer will return to the mound later this week.

Rivera hasn't pitched in a game since Aug. 31, because of a mild muscle strain near his right elbow. He got on a mound for the first time on Saturday, throwing 32 pitches in the bullpen at Yankee Stadium.

"He's getting a little antsy right now," manager Joe Torre said. "He's been in a lot more playful mood lately than he was for 10 days or so."

Rivera has insisted all along that he wasn't concerned about the injury, even though it came at a point in the season when he is typically tuning up for the postseason. Following his bullpen session on Saturday, Rivera seemed excited about how he felt.

"Even though he's had something similar to this in the past, he's never really had stop himself from doing anything," Torre said. "The last few days, his personality is a little more upbeat."

Gary Sheffield took the next step toward a move to first base, working out with Don Mattingly and a group of Yankees coaches before Monday's game at the Rogers Centre.

Sheffield worked on his footwork, his throwing and some overall knowledge of the position, with Mattingly -- a nine-time Gold Glove Award winner at first -- guiding him through each situation.

"He was really good," Mattingly said. "It looked really easy for him. He can do it; there's no question he can do it."

Joe Torre watched from the side, even offering some words of advice to Sheffield during the practice. Sheffield took ground balls with a ball in his right hand and his hand behind his back, a drill designed to concentrate on footwork that Mattingly picked up years ago from Eddie Murray. He also worked on turning double plays, making most of his throws with accuracy -- save for one that sailed into left field.

"He was a good student; he worked at it," Torre said. "This is something he needed to learn; it's not like taking extra batting practice. It's a new position for him, even though he's been an infielder. It seems like he's going to be comfortable."

Sheffield has played 468 games at third base in his career and another 94 at shortstop, so the infield isn't completely new territory for him. Mattingly believes that experience can only help make the transition easier for the 37-year-old.

"It's an advantage for him that he's played the infield," Mattingly said. "He's caught ground balls, he's moved both ways, so he understands it as opposed to a guy like Bernie who has never played infield in his life. It's not a foreign country."

Sheffield said he has talked with former All-Star first baseman Fred McGriff about the move, which McGriff called "a blessing in disguise" because it could extend his career.

Sheffield believes the position switch will enhance his value as a player, giving him more versatility. The Yankees hold a $13 million option on Sheffield for 2007, but if they decline to exercise it, Sheffield will be looking for a new team.

Torre hasn't decided when Sheffield will return to action, as he is scheduled to take part in a simulated game on Tuesday afternoon. Pitching coach Ron Guidry will pitch to Sheffield, who will also run the bases.

Mattingly believes that Sheffield will pick up plenty by doing drills, but in the end, he will have to learn on the job.

"He can work on all that stuff, but it's a matter of doing it in a game," Mattingly said. "You can only simulate so much. You can talk about everything, but a lot of things out there come from experience."

When asked if he expects to play in a game on this week's road trip, Sheffield said, "More than one, I hope."

"I want to get in there and see where I'm at," Sheffield said. "I know there's going to be a little rust. It's just one of those things where you have to do it."

If Sheffield is able to show the Yankees that he can handle first base, he may find himself there during some games in the postseason. Torre would like to use Jason Giambi as the designated hitter, enabling him to stay in the game in the late innings instead of being lifted for a defensive replacement.

"If we feel comfortable that he can play the position, I don't think that's an issue," Torre said of Sheffield. "But we have to see something that will make us feel that way."

Ron Villone's struggles continued on Sunday, as he allowed four runs in one inning, taking the loss against the Red Sox. Villone has appeared in 66 games this season, posting a 4.50 ERA, but his past seven weeks have been a battle for the left-hander.

In August, Villone tied Scott Proctor for the AL lead with 17 appearances. While Proctor put up a 3.27 ERA in the month, Villone's was a whopping 6.04.

This month, Villone has pitched in five games, allowing 12 runs in 3 2/3 innings for a 29.45 ERA. Torre said that the pitcher has no physical problems, so he'll keep sending him out there, hoping he can straighten himself out.

"He's still what he is. He's still a guy who will make great pitches, then make some other pitches," Torre said. "He's always been that way in his career. Talking to guys who have faced him, that's what makes him tough to hit. You can't zero in on one spot."