ST. PETERSBURG, Fla. — Arm strength among starting pitchers isn’t fully developed until they have worked 60 to 70 regular season innings. It has been a staple of the craft forever.

So, it wasn’t surprising to hear Masahiro Tanaka say he continues to build arm strength after two subpar starts, in which he has worked a total of nine innings. He starts Saturday night against the Rays at Tropicana Field.

“I was building arm strength right at the same time [last year] as well,’’ said Tanaka, who is 0-2 with a hefty 7.00 ERA. Last year, he was 6-0 with a 2.70 ERA in his first eight starts before suffering a loss.

But last year, Tanaka wasn’t coming off a small tear of the ulnar collateral ligament in the right elbow like he was this year. The Yankees worked him slowly in spring training (four games; 14 ²/₃ innings) to make sure the elbow was built for a full season after he appeared in just 20 games last season and only two — in September — after July 8.

“I don’t look in the past saying I should have thrown more in spring training,’’ Tanaka said.

Manager Joe Girardi said Tanaka’s velocity is down less than a mile per hour than where it was at this time a year ago.

“Is there more to come, I think there is,’’ Girardi said. “Is there a lot more to come? That remains to be seen. I wouldn’t think so. We have seen him as high as 94 in spring training. What’s the hardest we saw him last year, 95, 96?’’

Brett Gardner took batting practice, but wasn’t in Friday night’s starting lineup for the third straight game because of a bone bruise in his right wrist suffered on Monday night in Baltimore when he was hit by a pitch.

Inserted as a pinch-runner in the eighth inning after Carlos Beltran singled, Gardner stole second and scored on Alex Rodriguez’s single to center that accounted for the winning run in a 5-4 Yankees victory.

“I was prepared [to hit] if my spot came around,’’ said Gardner, who stayed in the game in left field and hopes to return to the starting lineup Saturday night.

“If the score was tied, he probably would have stayed in [to bat],’’ Girardi said. “I don’t know what that means [Saturday].’’

Ivan Nova said he was extremely encouraged with his second batting practice session Friday at the Yankees’ minor league complex in Tampa, where he threw 20 pitches.

“This one was better than the first,’’ said Nova, who is coming back from Tommy John surgery. “My curveball was not there today but my fastball, change-up and slider were right there with a little more intensity on the pitches.’’

Nova, who had the surgery last April 29, faced batters for the first time this past Tuesday and his next session is slated for this coming Tuesday.

“I am guessing but they gave me two weeks of bullpens and two weeks of BP and then games,’’ Nova said.

Though it has been less than a year since the surgery, and the recovery time can be from 12 to 16 months, Nova is getting excited about the prospect of returning to the Yankees’ rotation.

“I have been able to throw to two hitters, it feels awesome,’’ said Nova, who threw batting practice to Jose Pirela and Nathan Mikolas. “I feel like I am close to where I have to be and I believe I will be there soon.’’

Nova, whose 2014 season ended April 19 at Tropicana Field, will rejoin the rotation when he does return, which could be early June.

By then, the Yankees hope to have lefty starter Chris Capuano back from a quadriceps problem that had him open the season on the disabled list. Capuano is scheduled to throw batting practice at the minor league camp Sunday.

With the Yankees starting a stretch of 30 games in 31 days on Friday night, Girardi was asked about using more than five starters and said it has been discussed. He mentioned Bryan Mitchell and Chase Whitley, currently two-fifths of Triple-A Scranton/Wilkes-Barre’s rotation, as candidates.

He also said long reliever Esmil Rogers is a possibility, but that would mean Rogers wouldn’t be available in relief immediately after a start.

According to Girardi, utility infielder Brendan Ryan will start working out at the minor league facility next week. Ryan is on the DL with a calf injury.