SAN DIEGO — The Nets haven’t even hit their preseason opener before getting hit with injuries. 

Backup center Day’Ron Sharpe and rotation forward Trendon Watford will each miss several weeks with left hamstring strains, after sustaining their injuries during training camp practices last week. 

Both are sidelined from on-court activity, with Sharpe’s injury the more severe and a tough blow for the Nets. He won’t be updated for six weeks, with Watford expected to be updated in two. 

Sharpe and Watford join starting center Nic Claxton and wing Bojan Bogdanovic in missing Tuesday’s preseason opener versus the Clippers.

The Nets are expected to go with a small-ball lineup of Dennis Schroder, Cam Thomas, Cam Johnson, Dorian Finney-Smith and Ben Simmons, with Noah Clowney the only big off the bench. 

“Obviously next man up. But you just feel bad for those guys, because they’ve been having a great summer,” Finney-Smith said. “DayDay looks amazing, weight-wise, movement-wise. He was banged up with his left hand last year [and] as soon as he rehabbed that he comes and gets injured again. You feel bad for the guy, because I know he really wanted to be on the court. And T-Wat has been looking good. He’s been shooting the ball all training camp. 

“They’re going to be back. … So, it’s just next man up. So if I’ve got to play more five, I will. We’ve also got Noah who can play the five as well. So, we’re definitely deep enough to figure it out. But we need those guys back. So [I hope they] take care of their bodies.” 

Claxton missing Tuesday with a hamstring issue is being downplayed as purely precautionary, but Sharpe is the only other true center on the roster, so losing him for a minimum of 1 ¹/₂ months is significant.

The 6-foot-11, 265 pound bruiser is one of the league’s best offensive rebounders on a team in desperate need of size and strength on the offensive glass. 

Last season, Sharpe averaged 6.8 points and 6.4 boards in just 15.1 minutes backing up Claxton.

His 16.3 offensive rebound percentage is third-best in the NBA among all players who’d logged at least a dozen games last season. 

Watford (6-9, 242) had found a role late last season as a point forward. 

New coach Jordi Fernandez had already planned to experiment with going small, but having his frontcourt decimated has forced his hand. 

“We’re going to play smaller because of the three guys that are out. Nic, obviously he’ll be back soon, but it just forced us to do something that actually we wanted to do sooner or later; so might as well do it the sooner the better,” Fernandez said. 

“We have positional size. The lineup is going to be Dennis, CT, CJ, Dorian and Ben. That gives you a lot of things to do defensively. And then offensively, two playmakers and floor spacers. So it’s going to be fun. Obviously we’re not going to see a traditional center, because we don’t have it. So … that’s how we want to play — play fast, find high quality shots. Our guys have done a really good job so far getting there during training camp. So we’ve just got to do it during a real game.” 

Finney-Smith played some small-ball five at times in Claxton’s absence last season, and Simmons is listed at 6-10, 240.

Clowney is 6-10 and has added 20 pounds since last camp, up to 230. Meanwhile, on offense the Nets will essentially play five-out until Claxton’s return, which is presumably imminent. 

“Obviously it’s unfortunate guys go down, but it’s next man up,” Simmons told The Post. “It’s not ideal for any of us. I’m not sitting here trying to play the five. But I’ve got to do what the team needs. So right now we’re short some centers, so now we’ve got to figure that out. I’ve gotta run some five and get to that spot. So it’s gonna look different, but I’ve gotta do what I’ve gotta do for the team to make it work.”