When Zach Eflin went on the injured list last week with a right knee tendinitis, he mostly brushed off the move as precautionary and said he was “not overly concerned about it” lingering for weeks on end.

Is it time for the Phillies to get concerned?

Eflin threw a bullpen session Sunday in preparation to be activated this week. Instead, the right-hander will remain on the shelf, according to manager Joe Girardi, because his knee didn’t respond as well as he and the Phillies hoped.

“He’s not quite where we need him to be,” Girardi said Monday before the opener of a four-game series against the Washington Nationals at Citizens Bank Park. “It’s still not where we think it should be, so we will continue to keep going, see if we can get another bullpen in and see if it gets better.”

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Eflin has already missed two starts, including Monday night’s game. Asked if he will be ready in time to make his next start, scheduled for Saturday night in Pittsburgh, Girardi said, “No, I wouldn’t think so.”

The timing here is sensitive. The trade deadline is looming at 4 p.m. Friday, with the Phillies looking to upgrade their pitching. President of baseball operations Dave Dombrowski said over the weekend that the team was confident that “Eflin is going to be fine.” But if there’s greater uncertainty about Eflin’s status, the front office may feel compelled to prioritize a rotation addition over help for the bullpen.

As much as anything, the Phillies’ chances of winning the NL East are tied to their top three starting pitchers. Zack Wheeler, Aaron Nola, and Eflin stayed healthy through the season’s first half, each topping 100 innings before the All-Star break. They have combined for a 3.56 ERA in 58 starts. The back end of the rotation -- Vince Velasquez, Matt Moore, Chase Anderson, and Spencer Howard -- has combined for a 5.92 mark.

“If you’re going to go on a winning streak, which you need to do at some point to win [the division], that usually means you have good starting pitching,” Dombrowski said. “Going forward, that means we need Aaron and Zach Eflin to pitch well. And I think they’re capable.”

Howard started in Eflin’s place against the Nationals.

Unless the Nationals sell Max Scherzer (and if they do, it won’t be to the Phillies) or the Minnesota Twins move on from José Berríos, the trade market lacks difference-making starting pitching. Texas Rangers right-hander Kyle Gibson, a 33-year-old right-hander who was an All-Star this season but has a 4.42 career ERA, is among the top pitchers believed to be available.

See you in September?

A few days shy of the one-year anniversary of having Tommy John elbow surgery, reliever Seranthony Dominguez may go out on a minor-league rehab assignment by the weekend and could be ready to return to the Phillies in September.

“I feel really good right now,” Dominguez said. “I’m doing my best to get back and be here in September or at the end of August.”

Dominguez, who has spent the past few days with the team, said he has faced hitters “11 or 12 times” in a live-batting practice setting. His velocity has not yet fully returned, although that may occur when he gets into more competitive game action. He said his fastball has sat at 93-94 mph. Before the injury, he was averaging 97.5 mph.

“The simulated games aren’t the same as a regular season from a general standpoint, and I wouldn’t think a minor-league game would be the same as a big-league game,” Girardi said. “I believe that there’s more in there. I think it’s possible we could get him back [this season].”

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