Masahiro Tanaka has been struggling, especially when the Yankees face the Red Sox

BOSTON – When Masahiro Tanaka took the mound at London Stadium on June 29, he was headed for what appeared at worst a solid season and, at best, one of his better ones as a Yankee.

The righthander entered that sweltering afternoon against the Red Sox 5-5 but with a 3.21 ERA.

Tanaka then got annihilated, allowing six runs in two-thirds of an inning in the Yankees’ insane 17-13 comeback victory.

It was the start of a five-game stretch in which Tanaka has gone 2-1 but has an 11.69 ERA. He has allowed 33 hits, including eight home runs, and eight walks in 22 1/3 innings in that span.

A big portion of that, of course, was a result of Thursday’s disastrous outing at Fenway Park, when he allowed 12 runs and 12 hits in 3 1/3 innings in a 19-3 loss.

Tanaka, whose season ERA shot to 4.79 from 4.00, fell to 7-6. He has allowed 21 homers in 21 starts – he allowed 25 home runs in 27 starts last season after giving up 35 in 30 starts in 2017 – and has a career-worst 1.26 WHIP.

The numbers are extra concerning  because if the Yankees don't significantly upgrade their rotation before the trade deadline, Tanaka more than likely will be the Game 1 starter in any playoff series.

But there’s this, too: The two worst of those five outings both came against the Yankees' AL East rival. In his last four innings against Boston, Tanaka has allowed 18 runs (all earned), 16 hits and five walks.

“That’s something we have to dive into a little closer,” manager Aaron Boone said Thursday of Tanaka’s recent difficulties vs. the Red Sox, a team he’s had more success than not in his career, including getting the win in Game 2 of last year’s Division Series at Fenway,  when he allowed one run in five innings. “They’ve obviously been  on [his] pitches and  seeing him well. It’s something we’ll examine and see where we can eliminate that moving forward.”

Boone boiled down Tanaka’s specific issues Thursday to “a lot of mistakes in the heart of the plate,” but before Friday's game, he didn’t rule out the possibility of Tanaka tipping his pitches and/or the Red Sox stealing signs, something they excel at (and did to Luis Severino in Game 3 of last year’s ALDS).

“We have a lot of eyes on that and try to be very vigilant on that,” Boone said. “And obviously the Red Sox are very good at picking up any advantage you give them or seeing different things that can give you an edge, They’re very good at that. So it’s something that we try to stay on top of with our guys, obviously. So we’ll continue to be all over that as best we can.”

If Tanaka had an answer for his recent struggles against the Red Sox, especially Thursday’s performance, he wasn’t offering it publicly.

“I felt like the stuff wasn’t bad, the pitches weren’t bad, but it ended up what it ended up to be, so I do need to look at some stuff and figure things out,” Tanaka said through his translator. “I’m not sure at this point in time why I wasn’t able to basically get outs.”

Asked a few minutes later about the Red Sox, Tanaka said: “Like I said earlier, it’s hard to say why at this point in time, but it can’t be like that. Obviously, I’ll go back and see stuff and sort of make a determination on which way to go moving forward.”