NEW YORK -- Heavy rain did not provide the New York Yankees with any clarity about what is wrong with Masahiro Tanaka.

Two horrific starts prompted inquiries from fans, media and even from Tanaka himself.

After a rainout on Thursday, the Yankees will be curious to see if Tanaka can correct some, if not all, of the things going wrong on Friday night in the opener of a three-game series with the Oakland Athletics.

In his last two starts, Tanaka's ERA has risen from 4.36 to 6.56. He allowed eight runs and seven hits in 1 2/3 innings during a 10-7 loss to the Houston Astros May 14 and followed it up by allowing six runs and nine hits in three innings during Saturday's 9-5 loss at Tampa Bay.

"I can't really go too much into detail of what's going on, but yes, I was able to check to see of the things that I'm doing wrong," Tanaka said through a translator. "I feel like I'm in sort of a deep hole.

"Again, I just can't put my head down. I have to lift my head up, work on the things that I need to work on and try to fix what I need to fix. Definitely it's frustrating but I'm trying to get it right."

Since pitching a three-hitter in Boston on April 27, Tanaka has a 10.50 ERA in May and hitters are batting .395 against him. His worst ERA in any month with at least four starts was a 4.12 mark in six starts last June.

Tanaka has been prone to home runs, allowing 13 in his first 48 innings.

"As far as it being one of the worst slumps in my career, I probably have to agree with that," Tanaka said. "You have to grind it out. You can't put your head down. Physically, there's no problems at all. I feel fine."

The fact that it's not physical can be viewed as a good sign for the Yankees, who are optimistic it's just a rut and not a prolonged slump.

"I think as a coach you look for signs that someone is hurt," manager Joe Girardi said. "We haven't seen a real drop in velocity, we haven't seen him not being able to make a start. We haven't seen him receiving extra treatment.

"He's just in a little rut now that he needs to get out of. He feels fine. The schedule has been favorable for starting pitchers in a sense to get extra days. We've had a ton of off days, so maybe getting on every fifth day will help. Who knows?"

Tanaka is 3-0 with a 1.31 ERA in three career starts against Oakland. He faced the Athletics on May 21, 2016, in Oakland and allowed one run and five hits in seven innings, and on July 9, 2015, in New York, he allowed one earned run and two hits in 7 2/3 innings.

Kendall Gravemen (2-2, 3.83 ERA) starts for Oakland and also is looking to reverse some recent struggles. Since returning from a strained right shoulder on April 27, he is 0-2 with a 4.97 ERA and has allowed a .289 opponents' batting average in his last five starts.

Teams are hitting him well with the bases empty (.308) but are only 6-for-37 (.162) with runners in scoring position.

Graveman took a third straight no-decision last Friday when he allowed two runs and six hits in six innings during a 3-2 win over the Boston Red Sox decided on Mark Canha's 10th inning home run.

In three starts against New York, he is 1-1 with a 3.06 ERA. He allowed two runs and eight hits in 5 2/3 innings on May 19, 2016, in Oakland in a 4-1 loss, but April 20, 2016 in New York Graveman allowed one run and three hits in 6 1/3 innings of a 5-2 win.

The Yankees have won three of their last four games after Luis Severino pitched eight outstanding innings in a 3-0 win over the Kansas City Royals.

It was the first time a starting pitcher completed at least seven innings for the Yankees since Tanaka on May 8 in Cincinnati.

New York will be without Jacoby Ellsbury, who is on the seven-day concussion list. Ellsbury crashed into the center field wall on Wednesday tracking down a fly ball by Alcides Escobar and was diagnosed with a concussion and a sprained neck.

Oakland is 5-3 in its last eight games following a four-game losing streak May 12-15. The A's are among the major league leaders in home runs with 68 but also lead the majors with 46 errors.

Both things showed up in Wednesday's 4-1 win over the Miami Marlins, as Khris Davis slugged his 14th homer but the defense made three more errors.

"We know that defense is part of the equation," general manager David Forst told the San Jose Mercury News. "We've pitched well. We've hit homers. We haven't done everything, and defense is a part of that."

Davis was 4-for-24 with 10 strikeouts last season vs. the Yankees and is batting .171 this month. Jed Lowrie was 4-for-4 Wednesday and is 13-for-25 (.520) in his last six games following a 4-for-31 skid.

"Really consistent the whole year," Oakland manager Bob Melvin said of Lowrie.

Yonder Alonso did not play Wednesday because of an injured right hand/wrist contusion. He was hit by a pitch Tuesday but is expected to play Friday.