SAN DIEGO -- In the season's first four weeks, Drew Pomeranz had established himself as a worthwhile investment and the Padres' most reliable relief arm.

Now, they'll be without the veteran left-hander for at least a week -- and possibly more.

Pomeranz landed on the injured list on Friday evening with a left shoulder strain. He'd been experiencing shoulder tightness since his 18-pitch outing in Texas on Tuesday. The Padres haven't offered a timetable for Pomeranz's return, and his IL stint will be retroactive to that Tuesday appearance.

“The medical team made the call: We're going to go ahead and put him on the IL, give him the 10 days, even though he has been progressing a little bit,” Padres manager Jayce Tingler said. “They just thought it best to do that. Instead of trying to push through this and go, the idea that these 10 days off ... instead of rolling the dice a little bit, hey, let's get this right.”

It's merely the latest in a series of rough news for the Padres' bullpen, which has already lost Kirby Yates (bone chips in his right elbow) and José Castillo (left lat strain).

Pomeranz appeared set to fill in for Yates as the team's primary closer, and he'd pitched 10 scoreless outings while allowing just one hit and striking out 12. Now he'll join Yates on the IL, which doesn't bode well for a struggling Padres relief corps.

Entering play Friday night, the non-Pomeranz relievers in the San Diego bullpen had a 6.65 ERA. Without him available, the Friars blew ninth-inning leads on consecutive nights.

There's no readymade replacement for Pomeranz. Tingler said he'd likely play matchups with a lead in the ninth and Pomeranz unavailable. In the Padres' 4-3 victory Friday night, it was Emilio Pagán who got the call in the ninth, working around a two-out walk to pick up his first save with San Diego.

In a corresponding move, the club recalled right-hander Luis Perdomo from its alternate site.

The Padres did, however, get some (much needed) positive news regarding their bullpen. Castillo, who has missed the first month of the season with a lat strain, is set to throw off a mound in the coming days, per Tingler.

Castillo still isn't close to a return, as he'll need to throw several bullpen sessions, then pitch in intrasquad games at the team's alternate training site. But the Padres could sure use his electric left arm down the stretch -- especially if Pomeranz's injury is a serious one.