WASHINGTON — Until a few days ago, the Phillies didn’t know for sure if Zach Eflin would be able to pitch Sunday.

In hindsight, maybe he shouldn’t have.

After allowing four runs and lasting only two innings in a sweep-spoling, 9-3 loss to the Washington Nationals, Eflin conceded that his bruised right knee isn’t 100% and that it hindered his ability to push off the rubber.

But after being cleared last week by team doctors, he wanted to pitch. And interim manager Rob Thomson said the brevity of Eflin’s outing — 38 pitches, the fewest in his 114 career starts — was by design. The idea, as Eflin put it, was to pitch in “kind of an opener situation.”

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“I haven’t really been able to comfortably push off my back leg the last two starts,” he said. “I can still deliver a pitch. The last thing I want to do is have the bullpen cover nine whole innings.”

If that sounds strange, well, it definitely is. The Phillies lack starting pitching depth in triple A and can’t afford an injury to one of their five starters. It’s unusual, though, to send a pitcher to the mound if he’s even slightly compromised, especially for a game in June.

Eflin said the injury occurred when he twisted the knee while throwing to first base after making a barehanded play June 9 in Milwaukee. The knee stiffened in a start last Tuesday night, causing him to exit after six innings.

But doctors determined that there wasn’t structural damage to his twice surgically repaired patellar tendon. The Phillies diagnosed Eflin with a bruise of the “fat pad,” in the terminology provided to Thomson, underneath the tendon.

After throwing a between-starts bullpen session Friday, Eflin thought he would be able to give the Phillies at least three innings against the Nationals. And the Phillies, despite winning 15 of the previous 17 games, decided three innings of Eflin were better than any of the alternatives in the pursuit of an exceedingly rare five-game sweep.

“We knew going in that we were going to limit him,” Thomson said.

Thomson figured Eflin for about 50 pitches. Eflin didn’t get that far.

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The sinkerballer had trouble keeping the ball down, although he said it wasn’t connected to his knee problem. In the second inning, Eflin hung changeups to Luis García for a leadoff double and Maikel Franco for an RBI single. Three batters later, Juan Soto smashed a belt-high cutter for a three-run homer, the slumping star’s only extra-base hit of the series in 22 plate appearances.

Andrew Bellatti replaced Eflin for the third inning, with the Phillies in a 4-0 hole. It marked the shortest start of Eflin’s career.

“He wanted to keep going, actually. It was my decision to take him out,” Thomson said. “During the first two innings he was doing a lot of running around, covering first, backing up bases, and it looked to me like he was a little ginger walking around. So, I was just being careful with him. I didn’t want to put him in harm’s way.”

Eflin said he didn’t feel any worse for the wear. He said the knee actually felt better than in his previous start and expressed optimism that the bruise is “close to being over with.” Given five more days of treatment, Eflin believes he will be ready to take his next turn Saturday in San Diego.

Just in case, the Phillies may want to have triple-A lefty Bailey Falter ready.

Eflin has dealt with knee problems throughout his career. He had surgery on his right and left knees in 2016 and missed the final 2½ months of last season when he re-tore the patellar tendon in his right knee.

Even though the Phillies insist the tendon is in good shape, Eflin admitted there’s always a risk of altering his delivery to compensate for knee pain and injuring something else.

“That’s probably the biggest thing that we talked about,” he said. “And it happened a little bit last year when I was compensating a lot, had some arm stuff going on. But the arm’s in a really good position right now. I think that was the whole thought process behind going [only] three innings.

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“It’s just one of those things where we’ve kind of got to find that common ground of when to push it and when to back off it and let it heal itself.”

The Phillies also have to decide if the risk is worth not having to dip into their mostly unappealing depth options.

Knebel struggles again

Trailing 5-2 in the fourth inning after César Hernández’s RBI double sailed over Nick Castellanos’ head in right field, with two runners on and one out, Thomson turned to demoted closer Corey Knebel to shut off the Nationals’ rally.

No luck.

After an intentional walk to Josh Bell, Knebel walked Nelson Cruz to force in one run, then came back out for the fifth inning and gave up a two-run homer on a high-and-inside fastball to Franco, the former Phillies third baseman who finished 2-for-3 with two runs and three RBIs. The Phillies took Knebel out of the closer role last week after he blew his fourth save in 11 chances. Thomson was encouraged by Knebel’s outing Friday night, noting that he could rapidly regain a late-inning role.

But Knebel continues to have difficulty landing his curveball and commanding his fastball. He got only one swing-and-miss with his fastball and threw only five of 12 curveballs for strikes.