SAN DIEGO -- The Padres missed out on a chance to land a starter at the Trade Deadline, then saw their own starter surrender eight runs in a 9-4 loss to the Rockies at Petco Park -- but that wasn’t the worst of it on Friday. This was ...

On a rough night all around for the Padres, star shortstop Fernando Tatis Jr. injured his left shoulder while sliding into third base in the top of the first inning, though the club is optimistic the injury isn’t too serious.

Tatis attempted to advance when Rockies third baseman Ryan McMahon booted a ground ball. In an effort to avoid the tag, Tatis tried to slide away from the glove of shortstop Brendan Rodgers. Instead, he planted his left hand, then immediately grabbed for his left shoulder in obvious pain.

Manager Jayce Tingler said postgame Tatis suffered a left shoulder subluxation, a recurrence of the partial shoulder dislocation that he has sustained on three separate occasions already this year. Tatis will be re-evaluated on Saturday, but it’s possible Tatis could avoid a stint on the injured list altogether.

“It’s so tough to predict, without a lot of information at this point,” Tingler said. “I will say, for whatever reason, he’s had a phenomenal ability of getting back in there sooner than we’ve expected. But it’s tough to say.”

On a disheartening day, Tatis’ injury was the low point for San Diego. The Padres acquired outfielder Jake Marisnick in a trade with the Cubs, but they came up empty in their search for starting pitching at the Trade Deadline. In the meantime, their two primary division rivals, the Dodgers and Giants, pulled off a pair of blockbusters.

• Here is every Trade Deadline deal

Instantly, the Padres’ thin rotation was put to the test, and Ryan Weathers endured his worst outing as a big leaguer. He surrendered a grand slam in the first inning to Ryan McMahon and a pair of two-run homers in the fourth to Connor Joe and Brendan Rodgers. Afterward, Weathers took full responsibility for the poor outing, but offered a message of resilience after the front office’s decision to stand pat in the rotation.

“We’ve got five guys, and us five are going to win games,” Weathers said. “Tonight’s just a little blemish. It happens sometimes in baseball. … But we’ve got five guys who are ready to rock and roll every fifth day, and we’re going to get after it.”

As for Tatis, he missed a couple games in Spring Training with the injury, then landed on the IL for 10 days in April, before missing one further game in June.

The Padres, of course, can ill afford to lose Tatis, who is currently the clear frontrunner for National League MVP. He entered play Friday night hitting .290/.371/.650 with 31 homers and 23 steals.

“Obviously, our best team is with him on the field,” Tingler said. “Unfortunately, we’ve had to go some stretches without him this year. If that’s the situation now, hopefully it’s for a very short period of time.”

A hush fell over Petco Park when Tatis went to the ground in pain. He was on the ground for about a minute while he was evaluated by a team trainer. Eventually, he walked off the field under his own power and was replaced at shortstop by backup Ha-Seong Kim.

If there's any room for optimism, it might be this: After a similar scene in April, when Tatis sustained a painful dislocation on a swing against the Giants, he landed on the IL as a precaution. But he was essentially fully healthy within a few days and returned after a minimum stint. The Padres can only cross their fingers and hope for a similar outcome this time around.

“That’s exactly what we’re hoping for,” Tingler said. “Obviously, we’ll take a little bit more time with the trainers, with the doctors and we’ll just have more information tomorrow.”

When Tatis sustained his injury in April, the Padres said they were confident he could do no further damage by playing through it while undergoing treatment. But if the injury were to recur, they said, the possibility of surgery would be revisited. That seems very unlikely, as shoulder surgery would end Tatis’ season. The plan is for Tatis to continue working through the injury with daily treatment and strengthening exercises to help prevent future dislocations.

At the time of the April injury, however, general manager A.J. Preller noted, “There's players that will get through the season and then, in the offseason, will get the surgery.” Perhaps that’s a viable path from here for Tatis.