OAKLAND -- Jorge Posada was considered available only in an emergency for the Yankees against the A's on Monday due to a sprained left ring finger.

Posada was clipped on his glove hand by a foul tip off the bat of the Blue Jays' Fred Lewis during Sunday's 7-6 win over the Blue Jays at Yankee Stadium, forcing him to leave the game. With Francisco Cervelli serving as the Yankees' only other catcher, the club may need to summon a third backstop if Posada does not heal quickly.

"I think we'll know in the next 24 or 48 hours if we need to do something, if he can physically play through this or we need to possibly add a third catcher," manager Joe Girardi said.

Girardi said that Posada "could be a couple of days" away from game action. The 38-year-old has been nicked often this season, having spent two weeks of May on the disabled list with a hairline fracture of his right foot. He also missed time with right knee and calf issues in April.

"It's frustrating," Posada said on Sunday. "I'm feeling good at the plate and getting comfortable there. It's just one of those years."

Posada has worn a protective pad inside his glove to guard against foul tips, but the unpredictable nature of catching makes it impossible for him to be completely protected.

"The thing is, where there's a foul tip, the ball doesn't go where your eyes expect it to," Girardi said. "That's the hard thing to guard against."

Aceves' bullpen session cut short

OAKLAND -- Alfredo Aceves' chances of rejoining the Yankees' bullpen were dealt another blow on Monday, as the rehabbing right-hander was unable to complete a bullpen session at the club's complex in Tampa, Fla.

Aceves had been scheduled to throw 40-45 pitches off a full mound but felt more discomfort in his lower back, which has kept him on the disabled list since May 12. Aceves is being sent to New York to be evaluated by team physician Christopher Ahmad.

"From what I understand, he started his bullpen [session] and wasn't able to finish it," manager Joe Girardi said. "They're going to have to look at every alternative they have to see what's the quickest way to get him right."

This is Aceves' second setback as he attempts to return from the lower back strain, and surgery could now be considered an option. Aceves was 3-0 with a 3.00 ERA in 10 games for New York this year after going 10-1 with a 3.54 ERA in 43 games (one start) in 2009.

"He was extremely valuable for us last year, as anything from a one-inning guy to a three-inning guy that could bridge the gap in a lot of different ways," Girardi said. "People have a chance to step up, and we'll see what happens."

Worth noting

Yankees right-hander Sergio Mitre was scheduled to pitch an inning on Monday for Class A Advanced Tampa, but rain canceled the outing. Mitre has been on the disabled list since June 15 with a strained left oblique. ... The Yankees' 50-31 start is their best since they were 51-30 in 2004. ... Javier Vazquez is making his 400th career start on Monday, making the Yankees the only team with two active pitchers to have made 400 or more starts. Andy Pettitte, with 474 career starts, joins Vazquez. ... All-Stars Robinson Cano and Derek Jeter are set to join Willie Randolph and Bucky Dent (1980, '81) as the only Yankees second baseman-shortstop combination to start a Midsummer Classic together. ... Yankees pitching coach Dave Eiland turned 44 on Monday.