The 26-year-old Hill has been bothered by a sore right forearm this spring and threw on the side Monday.
"I don't think Shawn is going to be ready to leave with us (and come) up north," Nationals manager Manny Acta said. "It's out of the question to me right now."
Coming into spring training, Hill was battling for the top spot in the starting rotation. He made 16 starts for Washington last season, finishing 4-5 with a 3.42 ERA. He had surgery on his right forearm, right elbow and left shoulder in November.
Acta said the absence of Hill could open a spot in the rotation.
Valentin is hoping the shot is all he needs to get better but could retire if doctors recommend surgery.
"I'm going to be out again for three more months, come down there and get rehab?" Valentin said in Port St. Lucie. "By that time, it's already four months into the season. What do I have to go down there and prove? Nobody's going to give me a contract for two months just in case somebody got hurt."
The 38-year-old Valentin, a non-roster invitee in camp on a minor league deal, was hoping to make the team as a utility player. He already has been recovering from a surgically repaired knee, but that injury appeared to be healing in time for opening day.
Perez allowed two hits and one unearned run against the Nationals. He struck out five and walked none.
Jose Reyes went 2-for-3 with a pair of stolen bases and David Wright had two hits and two RBIs for New York, which finished with 11 hits.
Washington starter Mike O'Connor gave up four earned runs and seven hits in 4 2/3 innings.
