Heading into 2023, the Cubs’ rotation — and particularly their starting pitching depth — was praised for being much deeper than the previous year. For the first time this season, that depth will be put to the test.

Jameson Taillon became the first Cub to hit the injured list in-season in 2023. Slated to start the series opener against the Dodgers on Thursday, Taillon instead was placed on the 15-day IL with a “mild to moderate” left groin strain before the 6-2 loss.

Cubs manager David Ross said Taillon felt something in his groin Wednesday while playing catch in Oakland. He did get some treatment on the leg and it loosened up, but after some time, it tightened back up. Rather than let him try to push through it, the Cubs opted to put him on the IL.

Ross said they’re hopeful Taillon’s stay on the IL is minimal. With the team backdating his IL stint to Monday, that means Taillon would be eligible to come back on May 2. If the Cubs keep a normal five-man rotation, that would come a day after what would be his turn. The Cubs could play around with the starts — like for instance, utilizing Monday’s off day to skip Taillon’s next turn in the rotation — to keep the need to fill in at a minimum. It probably won’t be until the end of this series that Ross finalizes his starter plans for when the Padres visit Wrigley.

Regardless of how the Cubs go about it, this is why they didn’t only go out and sign Taillon for $68 million over four years. They built up their starting pitching depth, too.

Injuries forced them to use 17 different starters throughout the season in 2022, and at least before the All-Star break, they didn’t have the quality depth to help them weather that storm.

“You never want to have injuries, but this is part of what we have to deal with,” Cubs president of baseball operations Jed Hoyer said Thursday. “There’s always going to be someone banged up or there’s always going to be guys getting hurt. Last year, we didn’t have enough depth to withstand that.”

But the Cubs added or kept enough pieces where they have mostly dependable starting pitching more than five deep. Beyond Taillon, Marcus Stroman, Justin Steele, Drew Smyly and Hayden Wesneski, the Cubs have pieces they feel confident moving into the rotation if the need arises.

Javier Assad was the first of those pieces to be utilized when he was recalled from Triple-A Iowa to take Taillon’s place on the active roster. Between his time with the Cubs in spring training and with Team Mexico in the World Baseball Classic, Assad built up to be a multi-inning reliever, and his success in March earned him an Opening Day spot in the Cubs’ bullpen. He struggled out of the gate, however, and was optioned to Iowa after just two outings.

That also did allow for Assad to join Iowa’s rotation and get stretched out to start again. He got made his first start last Friday, which saw him throw 47 pitches over 2 2/3 innings. Of course, the Cubs planned on him getting a few more outings to build up for a starter’s workload. Unfortunately, those plans changed when Taillon went on the shelf, and Assad instead made his first start of 2023 Thursday.

It wasn’t a stellar outing by any means, but the only damage the Dodgers made on the scoreboard came from solo shots by Max Muncy and James Outman. Still, a little too much traffic and a few deeper counts meant Assad had to throw 62 pitches to get through three innings, which ended his night earlier than they hoped.

“I was trying to get as much out of him as I possibly could,” Ross said. “He hadn’t been stretched out enough since he left here to go much further than the 60s. I didn’t want to send him back out there. Didn’t look like he was his sharpest today, and just was laboring pretty hard in some innings. We got all we could out of him.”

For now, it seems like Assad will be the fill-in starter while Taillon is sidelined. But if his IL stint happens to be longer than the minimum stay, that would leave room for another arm to rejoin the rotation.

Kyle Hendricks is nearing a return from the capsular tear in his right shoulder that ended his season with three months to go in 2022 and has so far delayed his 2023 debut. He has been in Arizona building up with live BPs, bullpens and simulated-game outings. He’s scheduled to throw three innings in an extended spring game on Saturday, and a rehab assignment might be the next step provided everything goes well.

The plan for Hendricks following his shutdown was to build himself back slowly, with a plan built around altering his arm path for a shorter, more efficient delivery. Hendricks’ plan also involved velocity drills to help him regain a couple ticks on his fastball. Hoyer noted that his velocity had dropped in recent years, which hurt his overall effectiveness. So, they made that a focus of this lengthy process.

“We wanted to push his velocity up as much as possible,” Hoyer said. “When he was winning ERA titles and those things, he was touching 90 [mph]. That had come down a little bit. He’s not [A’s rookie] Mason Miller throwing 102 miles an hour, but when he throws 88-90, his changeup is that much more effective.”

Outside of a couple bad outings, the Cubs’ rotation had been performing among the elite. The starters had a combined 2.66 ERA going into Thursday, the lowest in the National League. The group also owned MLB’s lowest hard-hit rate (31.1 percent) and sixth-lowest WHIP (1.15). It was the kind of early-season display that showed the Cubs’ belief in their starting pitching wasn’t just talk.

With Taillon hitting the IL, though, the starting pitching depth will now be tested. That depth was one of the reasons both the Cubs and Hendricks felt comfortable taking their time with his recovery process, and it might just help them feel OK with not rushing Taillon back. But the depth pieces have to perform to make sure that sentiment doesn’t shift.

“This year, we have guys like Javy. Hopefully Kyle comes back soon. Hopefully we’ve built up some more depth,” Hoyer said. “Injuries can never be an excuse, right? So, you just got to have guys ready to fill those spots.”