When Mark Teixeira’s left calf underwent an MRI exam Sept. 10, the prognosis was that he would be out 10 to 14 days.

However, that’s not going to happen. The 10th day is today and the 14th is Sunday. Now, the Yankees are looking at a four-game series in Toronto (Sept. 27-30) to get back their switch-hitting first baseman.

“I wouldn’t be surprised if it took that long,’’ manager Joe Girardi said. “As of Sunday he still had soreness. It’s not progressing as quickly as we would like. We have to get him right.’’

Teixeira exited a game on Aug. 27 with a calf injury. He missed 10 games, returned for one on Sept. 8 in Baltimore and re-injured the leg on a head-first slide into first base that ended a 5-4 loss.

Before Teixeira came back for one game, the Yankees were 4-6. Since he played against the Orioles, the Yankees are 5-2.

The latest development presents a dilemma for the Yankees. If Teixeira returns next Thursday, that leaves him seven games remaining, at the most, before the postseason begins. If he doesn’t come back until a week from Sunday, that’s four games left.

Is that enough time for Teixeira to find a stroke after missing what would be about a month? The Yankees likely would take a less-than-100 percent Teixeira because of his presence in the middle of the lineup.

With Teixeira out, Girardi has used switch-hitter Nick Swisher at first against right-handers and Steve Pearce versus lefties.

Last week in Boston, Swisher showed signs of coming out of a horrific slump. He entered Tuesday’s game hitless in 28 at-bats and with four hits in 51 (.078).

Seven hits in 10 at-bats followed, but since then Swisher is hitless in 10 at-bats. In addition he is 0-for-12 with runners in scoring position and has one hit in the last 16 clutch at-bats.

george.king@nypost.com