PHILADELPHIA — Bryce Harper woke up Saturday morning with soreness in his lower back, keeping him out of the Phillies’ lineup against the Cardinals at Citizens Bank Park.

The Phillies said Harper is day-to-day, but he suffered a similar injury last season, which is cause for concern.

“It’s something that we have to watch and stay on top of,” Phillies manager Joe Girardi said on the Phillies’ pregame show on 94 WIP. “I told him, ‘Look. Let’s get this right. Let’s not try to fight through this because we have such a long season. Last season was a sprint. This season is a marathon and we want you feeling good when you’re out there.’”

Girardi said Harper might have injured himself sliding headfirst into second base on a double Friday night.

“He wakes up today and he’s sore,” Girardi said. “Once you have some back issues, you’re always a little bit concerned about it. Hopefully it is short-term.”

Harper suffered back soreness the final month of last season. He said in February in Clearwater, Fla., that an MRI exam in the offseason revealed no structural damage, but an issue with the QL (quadratus lumborum) muscle.

“When I talked to the doctor about it, he was kind of just, you know, ‘You have no herniation, your spine is great, your discs look amazing,’” Harper said then. “I had something going on with my QL attaching to my oblique area. So when I threw or when I hit or when I stretched it would pull, and then when I hit it would pull as well. So once we figured that out and kind of got the MRI done with it and got everything done, got everything flushed out of there, I was good. But all my discs, I have no herniation or anything like that.

“So this is something where, hopefully if I keep working out, do what I need to, keep staying on top of my program and things like that throughout the whole year, then it might maybe be a little bit sore, but it won’t affect me like it would last year. So I don’t think this is a long term thing. Knock on wood.”

Harper had a 1.147 OPS in 21 games last season before the back started to bother him on Aug. 21. He had an .860 OPS the rest of the way. The discomfort got so bad at one point that he removed himself from a Sept. 20 game. He served as the Phillies’ designated hitter in five of his final seven games.

To help his back this winter, Harper started to hit a month later than normal (from Jan. 1 to Feb. 1).

Harper this season is batting .238 with two home runs, six RBIs and an .825 OPS in 13 games. He might have had his best night at the plate in Friday night’s 9-2 victory over St. Louis. He hit a double 103.1 mph in the second inning, according to Statcast. He crushed a pitch 109 mph on a flyout to the warning track in right in the first. The ball had an expected batting average of .980. He crushed a pitch 108 mph on a flyout to the warning track in left-center field in the fourth. That ball had an expected batting average of .990.

Since Statcast began tracking exit velocity in 2015, Harper has had only one game in which he hit more balls in play harder than he hit Friday. He hit four 103.1 mph or better on April 19, 2019.