Are Cubs’ pitching-depth issues starting to wear thin?

Cubs manager Joe Maddon says he’s not concerned.

But the Cubs’ rotation – the backbone of the Cubs’ success this year — has suddenly slipped a disk with the most important six weeks of the season left to play.

Jason Hammel hasn’t found his form since a July leg injury. Kyle Hendricks has gone back to Class AA video to fix a delivery flaw he’s battled all season. Newcomer Dan Haren is an 86-mph, hold-your-breath-for-five-innings proposition.

And even as dominant as Jake Arrieta has been all season, he’ll be adding to a career-high innings total every time he takes the mound the rest of the way, which Maddon admittedly is monitoring.

And never mind Jon Lester’s so-called yips, that newly exposed inability to make a routine throw to a base.

One start after he survived the Brewers running at will for five steals against him, Lester had his worst outing of the season Wednesday night against the Detroit Tigers, lasting just 2 2/3 innings and giving up seven runs on three homers – including a solo shot and grand slam by Nick Castellanos in back-to-back innings.

The Tigers won 15-8 – combining for 25 runs and 40 hits in their two-game sweep.

Just like that the Cubs have lost three straight after winning 15 of 16.

And just like that, the fourth-ranked rotation in the National League has allowed 22 earned runs – including 10 home runs – in just 20 1/3 innings the last full time through the rotation. Hammel and Lester combined for 5 2/3 innings against the Tigers.

The starting crew had a 3.36 ERA this season before that turn.

“If there were health issues I’d be more concerned,” Maddon said. “If it was something like, `My shoulder’s barking a bit’ – that’d be my greater concern.

“During the course of a year guys are always going to go through some struggles. I think Jason’s very fixable. I think Kyle’s very fixable. And I’ve only known [Haren] briefly, but this guy’s a tremendous competitor, so I have a lot of faith in him, too.”

One time through the rotation certainly is a small sample size. And even within that, Arrieta continued his dominant summer with a strong start in a win Saturday.

If anything, the time of the year and the Cubs’ precarious position in the standings have heightened the stakes of each week, each game – and the scrutiny on a slumping position area that has been a depth question all year.

Even without a starter from the opening rotation going on the disabled list this season, the Cubs’ front office recognized depth as a serious enough issue to add Haren at last month’s trading deadline.

On Wednesday they took advantage of former All-Star Trevor Cahill’s availability to add the long struggling right-hander as potential change-of-scenery depth on a minor-league free agent deal.

But if it comes to that, it’s probably too late.

Hendricks, who has had two productive bullpen sessions since his last start, recognizes the urgency.

“It’s huge. I need to figure this out,” he said. “And it makes it that much more exciting that we’re onto something. That relief is kind of heightened because of what’s at stake.”

If the starting pitching is starting to crack, Lester might be the least of the Cubs’ worries.

In his previous eight starts, the left-hander had pitched like the $155 million ace the Cubs signed to that big free agent deal in December, with a 1.92 ERA.

“I’ve been down this road a time or two. I’ve had shorter outings,” said Lester, shaking his head at the way the Tigers were able to handle some of his better pitches on a rare night of hitter-friendly wind.

“The season’s too long and it’s too much of an up-and-down [thing] to allow one of those abomination starts that stick out to affect your season,” he said. “Obviously, this is a bad time of the year to have one, and it’s a bad time especially after [Tuesday’s bullpen-heavy game].

Wrigley Field played like a homer haven with the wind blowing out the past two nights, but neither Maddon nor Lester was willing to blame the lambasted pitching on that.

“No matter what I say, it doesn’t justify the outcome,” Lester said, stressing the value of “flushing” Wednesday and preparing for his next start. “I feel like within that game there were pitches that were made that deserved better results than what they were.”

For now it’s not going to do the hyper-taxed bullpen any good the next few days against the Braves. And the schedule doesn’t get any easier the rest of the way.

“I really think we can work through the issues,” Maddon said.