Yes, the bullpen has been short an arm, and yes, the black hole in the fifth starter spot often has crushed team momentum. But no pitching problem for the Red Sox has been more vexing than the utter mediocrity of a starting rotation that was expected to rank as one of the game’s elite.

When the Red Sox made the four-year commitment to Nate Eovaldi in December, it was with a clear vision in mind. Some teams structure their staffs around bullpens. Not the Red Sox.

“We built this team from the first inning through the sixth, and then the last three,” said manager Alex Cora. “There are other teams that build from the ninth all the way through the first inning. For us to do the things we want to do, our starters have to give us quality starts and we go from there.”

That hasn’t been happening. Quite the opposite. The Red Sox rotation this year entered Thursday’s game against the Blue Jays with a 4.67 ERA, 18th in the big leagues. The team is 18th in quality starts with 33.

With roughly $90 million invested this year in David Price, Chris Sale, Rick Porcello, Eovaldi, and Eduardo Rodriguez, this was not what the Red Sox expected. Even with Eovaldi making just four starts — and his replacements averaging roughly three innings and a 6.75 ERA in 16 starts — the rest of the rotation has been largely underwhelming.

“We built our ballclub for them to be our strong suit,” said president of baseball operations Dave Dombrowski. “I don’t think it’s been what we expected or hoped.”

David Price has been the team’s most consistent starter, forging a 6-2 record and 3.33 ERA in 15 starts — though, with some careful management of his workload, that has limited him to 78 innings. Chris Sale admitted to being lost in a first-half wilderness, going 3-8 with a 4.04 ERA. Eduardo Rodriguez has been typically inconsistent (8-4, 4.79), while Rick Porcello (5-7, 5.07) has navigated a dramatic rollercoaster.