The pang came as Kershaw progressed to throwing a little further, from the roughly 50 feet or so at which he started playing catch to roughly 100 feet.
"He felt it that last throw, and at that point we knew he couldn't go any farther," Mattingly said.
"I'm not going to get into specifics," Kershaw said. "I can't pitch right now. It's frustrating. I've been hurt before where I knew when I'd be able to pitch. I'm getting better right now, just not fast enough."
On Wednesday, Kershaw was scratched from starting Sunday night's domestic opener because of an inflamed teres major muscle, which stabilizes the upper arm near the upper back. Now, he also will miss his projected start for the home opener against the Giants on Friday.
While the Dodgers maintain they are not concerned, they also know the ceiling for Kershaw's current workload when they see it. And they believe placing him on the DL is the smart thing to do.
Especially with four off-days within this portion of the schedule's first 13 days, and no fifth starter needed until April 19.
"We've got to save Clayton from Clayton at this point," Mattingly said. "If it was up to him, I'm sure he'd want to go farther [in working to come back quickly]."
"We're going to protect him from him, and be cautious."
The move means that Hyun-Jin Ryu, slated to start Sunday's mainland season opener against the Padres, will also slide into the home opener start on Friday. Then, Josh Beckett likely will start against the Giants on Saturday.
"You don't feel like you're a part of the team when you're hurt," Kershaw said. "It's not a good feeling."
Kershaw would be eligible to come off of the disabled list early the following week -- the move is backdated to March 23 -- but the Dodgers are making no promises about when he will pitch again. Mainly, because there is little urgency in the season's first few weeks given the spate of off-days, and because they want to remain ultra-conservative and make sure Kershaw is right when he returns.
Tests earlier this week showed the strained upper back muscle, behind his left shoulder. Mattingly said he does not know of any more tests scheduled as of now.
"We know what it is," he said. "We're not concerned that he's going to do any damage to it."