Prized Mets pitching prospect Zack Wheeler, who had to deal with a blister earlier in the season, had another rough outing Friday night for Triple-A Las Vegas. He allowed just three hits against Sacramento, but coughed up four earned runs in 4 1/3 innings due to six walks. His ERA rose to 4.91, with the walks raising a red flag. “When you’re pitching in Las Vegas, due to the ballpark, sometimes you overthrow just because you don’t want to get hit,” manager Terry Collins said before the Mets’ 7-6 loss to the Nationals Saturday at Citi Field. “Sometimes guys in those parks pitch away from contact because contact means danger in those little parks. The ball flies. Zack’s history is he doesn’t walk a lot of guys, so when I saw the six bases on balls there’s got to be a reason for it. “He’s got good enough stuff. If he pounds the zone, he’ll get people out. ... That’s a red flag. I don’t want to see walks from those guys. I told Zack in the spring, ‘You’re going to pitch in a tough place.’ I was in that league for 12 years. I know how hard [it] is to pitch in, but good pitchers [have done well] because they threw strikes. I’d rather see 10 hits — that I can understand. But six walks, he’s better than that.’’