Chris Stewart was in the middle of all the big defensive plays in Saturday’s 5-2 win over the Red Sox at Fenway, but none stood out like his play in the eighth inning that helped protect a two-run lead. Stewart hung over the wall between the third base dugout and the screen — into the Boston crowd — and caught Dustin Pedroia’s pop-up. He then turned and fired a strike to Robinson Cano to nail Boston’s Daniel Nava trying to advance from first. It ended the eighth and left the tying run — David Ortiz — in the on-deck circle. Neither Joe Girardi nor bench coach Tony Pena — who have 33 years catching in the majors between them — could remember someone making that play. Stewart’s concentration in making the catch, his awareness of the baserunner and his long throw made it a special play. First baseman Lyle Overbay and players in the dugout were calling out, but as Girardi said, “You can yell all you want. He’s never going to hear you. . . . It was instinct.” “It’s just court awareness and it was fun to be a part of it and watch it,” Overbay said. Stewart was less stunned by the result of the play than how he was able to glove the pop-up in the middle of the Red Sox fans. “It seemed like a clear spot. I was surprised I had such a clear path to the ball,” Stewart said. “I jumped in there and didn’t see anyone trying to get in there and go after it. I was able to catch it because of that and I was able to hop out quickly too. It’s all reaction and instinct. I tried to pick (Nava) up.