SAN DIEGO — A bad start to the Phillies’ road trip at Dodger Stadium took a turn for the worse Monday night at Petco Park. In addition to losing their fifth straight game, an ugly 8-2 setback to the San Diego Padres, the Phillies may also have lost leadoff man Andrew McCutchen to a left knee injury for an extended period of time.

Manager Gabe Kapler described the injury as a sprained left knee after the game, but both the manager and McCutchen admitted they will not know the exact extent of the damage until the outfielder gets the results of an MRI examination that was scheduled for Tuesday.

The Phillies were concerned enough about McCutchen that they are flying 2017 first-round draft pick Adam Haseley into San Diego in time for Tuesday night’s game. Haseley, a center fielder, was recently promoted from double-A Reading to triple-A Lehigh Valley. He was hitting a combined .272 with seven home runs and 23 RBIs in 46 games this season.

“I didn’t feel a pop or anything,” McCutchen said. “Something that felt uncomfortable, something that made me grab my knee and go down. I have had an ACL injury before. I know what it feels like.”

McCutchen said this did not feel exactly the same as the torn ACL injury he experienced as a high school football player in Florida, but he admitted “it did not feel good.” The outfielder left the clubhouse on crutches and posted a tweet asking for prayers shortly afterward.

The mood in the manager’s office was glum.

“Very difficult,” Gabe Kapler said when asked how much it would hurt the Phillies if McCutchen was sidelined for an extended amount of time. “Andrew is not just a catalyst at the top of our lineup, he’s a catalyst in our clubhouse. He sets a great example for the rest of our club.”

McCutchen opened the game with a nine-pitch walk, his league-leading 43rd of the season. It appeared as if he would remain stationed at first base when Jean Segura hit a popup toward second baseman Ian Kinsler. Segura, however, slipped and fell in the batter’s box and then did not run full speed to first base.

Seeing an opportunity for a double play, Kinsler let the ball drop in front of him and threw to first base to record the out on Segura. McCutchen, caught in no man’s land, got stuck in a rundown and appeared to injure his left knee just before he was tagged out at the completion of the Padres’ unconventional double play.

“Jean and I discussed it after the game," Kapler said. “He did slip out of the batter’s box. He didn’t run his hardest out of the batter’s box. He owned that. He was an absolute professional about it. He didn’t make any excuses about it. He says it won’t happen again.”

Said Segura: “No excuse. Not acceptable. It was a mistake.”

The night went mostly downhill for the Phillies from there.

They did take a 1-0 lead in the fourth when Rhys Hoskins doubled and scored on a single by J.T. Realmuto, but that lead quickly disappeared, even with Aaron Nola on the mound. The Phillies’ top starter escaped a first-and-third jam with nobody out in the third, but allowed a game-tying home run to Franmil Reyes in the fourth on a change-up that he left up in the zone.

Nola recorded just a single out in the sixth when the Padres scored seven times on five hits to put the game away. Manny Machado’s first grand slam in a San Diego uniform, off rookie reliever J.D. Hammer, capped the Padres’ big inning. Bryce Harper had an up-close view as the ball cleared the right-field wall.

Nola (6-1) was charged with six runs on eight hits in 5⅓ innings, marking the third time this season he has allowed five runs or more in a start. A year ago, when he finished third in the National League Cy Young Award voting, he did not allow five earned runs in any of his 33 starts. Nola’s earned run average went from 4.18 to 4.63.

The Phillies’ lead in the NL East slipped to a half-game over Atlanta, and it will be difficult to maintain their 40-day stay at the top of the standings if McCutchen is sidelined for an extended period.

One of the many things the Phillies loved about McCutchen when they signed him to a three-year deal worth $50 million during the winter meetings was his durability. The 32-year-old outfielder had played at least 146 games every season since 2010 and at least 153 games in eight of those nine years. He had been placed on the disabled list just once in his career during the 2014 season with a rib fracture.

That will remain his only trip to the disabled list because baseball has changed its terminology this season to the injured list. Regardless of what it’s called, the Phillies’ lineup would be significantly weakened without McCutchen at the top of their order for an extended period of time.

“Just that thought (of the injured list) is disheartening,” McCutchen said. “It stinks. I like being out there. The last thing you want to be is hurt.”

McCutchen entered play Monday with a .375 on-base percentage, the highest among all qualified NL leadoff hitters. His .832 OPS was second among the league’s leadoff hitters and his 45 runs scored were the most among leadoff hitters in all of baseball.

The Phillies, of course, are already without center fielder Odubel Herrera, whose administrative leave after his May 27 arrest in Atlantic City was extended Monday until at least June 17, the scheduled date of his court appearance.

Kapler’s immediate solution to McCutchen’s departure was to move Sean Rodriguez from left field to center field and insert newcomer Jay Bruce into left field for his Phillies debut. Bruce did get his first Phillies hit, an eighth-inning single. If McCutchen is out for a while, Bruce can expect to be in the lineup quite frequently in the coming weeks.