BOSTON -- The curious case of Luis Castillo continued on Tuesday in perhaps his most puzzling start yet since joining the Mariners ahead of last year’s Trade Deadline.

Castillo surrendered a career-high-tying three homers and seven runs (five earned), which dug too deep of a hole for the Mariners’ bats to climb out of in a 9-4 loss to the Red Sox at Fenway Park.

“La Piedra” now finds himself in his first cold spell with the Mariners, as they’ve now lost each of his past four starts -- and the common theme in this stretch has been the long ball. No hitter took Castillo deep in his first five starts, yet they’d left the yard in each of his three outings leading into Tuesday, and then the trifecta at Fenway.

First 5 starts: 4-1 team record, 1.52 ERA, 0 HR, .170/.214/.217 (.431 OPS) opposing slash line
Past 4 starts: 0-4 team record, 5.73 ERA, 6 HR, .278/.323/.600 (.923 OPS) opposing slash line

It leads to the grander question of: What’s going on with one of the game’s best pitchers?

A lot of hard contact

The homers correlated to the obvious rise in hard-hit balls, but it wasn’t just those that left the yard on Tuesday. Boston blasted 11 of its 16 batted balls against Castillo beyond Statcast’s hard-hit threshold of 95 mph, one shy of his career high and more than any he’s had with Seattle. Six of those went for outs, including five with an expected batting average over .300, illustrating that he may have dodged even more damage.

Castillo entered the day ranked in the 18th percentile in hard-hit rate, way down from the 53rd percentile he was in last year. It’s worth noting that his opponents in this four-start stretch have included some of the hardest-hitting teams: Toronto, Texas and Boston, the latter of which packed its lineup with seven lefties.

“He can usually make adjustments mid-game, and I thought he kind of got it rolling,” Mariners manager Scott Servais said. “That's a tough lineup and in this ballpark. ... They were on him tonight.”

A lot of over-the-plate pitches

Had Castillo dominated, the conversation would’ve probably centered on the uptick in most of his stuff. He generated six punchouts, including three via gnarly sliders.

The velocity was also significantly up on both his fastballs, by 1.4 mph on the sinker and 1.5 mph on the four-seamer, notable given that he’s typically slow to grow into his velo each year. He also retired 10 of 11 after the four-runs in the first and before the three-spot in the fifth.

On the other hand, much was left over the plate, and as such, ambushed by one of MLB’s more aggressive-hitting teams, underscoring that even the most overpowering pitchers must locate and execute.

“He's one of those guys that he gets away with a once in a while because he is deceptive and his delivery is so different,” Servais said. “But against a quality ballclub like that with all the left-handed bats, you can't make too many mistakes in the middle.”

A lot of first-pitch balls

Castillo was most susceptible early in counts in the first inning, with Boston ambushing him on the first or second pitch for three hits, a triple from Masataka Yoshida and homers from Justin Turner and Triston Casas.

Then in the fifth, Castillo fell behind against five of his six hitters. Overall, he threw first-pitch strikes to just 13 of the 24 batters he faced, which forced him back over the plate once the Red Sox were in favorable hitters counts.

“Unfortunately in that fifth, I fell behind in the count a little bit and they were able to make me pay,” Castillo said.

A lot of similarities to last year

Castillo had a comparable stretch last year with Cincinnati, also in his second full month, when the Reds lost four straight starts by him, when he had a 4.06 ERA and hitters were slashing him .223/.315/.351 (.666 OPS). It’s possible that maybe even the best are mortal for brief blips.

“I feel great,” Castillo said. “Like I said, not every night is the same. I feel like every time I go out, I try to give my 100 percent with what I have. But the most important thing right now is that I thank God that I'm healthy.”