What happened during Friday’s 104-94 victory over the Boston Celtics is a perfect example of why Shawn Marion is so valuable to the Dallas Mavericks. Before scoring 11 points and collecting a game-high 13 rebounds in his return against the Celtics, Marion missed the previous eight games while nursing a strained left calf. The Mavericks went 5-3 in Marion’s absence, but the losses to the San Antonio Spurs (92-91), Oklahoma City Thunder (107-101) and Brooklyn Nets (113-96) were extremely painful, because the players who were largely responsible were the players Marion normally would have been guarding. Especially in the losses to OKC and Brooklyn. Marion normally guards Kevin Durant, who scored 19 of his 31 points in the fourth quarter to lead the Thunder to a March 17 victory over the Mavericks. Marion also would have guarded Deron Williams, who scored 26 of his 31 points in the second half when the Nets defeated the Mavs. Marion is the Mavs’ best defender, and he likely would have had an effect on Durant and Williams. “We miss his abilities as a basketball player, for sure,” coach Rick Carlisle said of Marion. “He’s one of our best athletes. He didn’t have a big scoring game [against the Celtics], but the rebounding was huge for us.” Marion, who will be in the starting lineup again at 6:30 Sunday night when the Mavs (33-36) host Utah (34-35) at American Airlines Center, was more than happy to get back on the court during this stretch of critical games for the Mavs. “It was frustrating sitting on the sideline wishing I could be back out there,” Marion said. “I’m glad to come back and get my wind in me a little bit. My timing is off a little, but it’s OK.” While admitting his left calf is not 100 percent, Marion wanted to come back and test it as soon as he could. “It’s one of those injuries where all you can really do is rest it and ice it and let it heal,” he said. “It limits you — you can’t run, you can’t jump, you just have to wait it out. “It’s frustrating, but at the same time it’s part of basketball. The last few days it’s been feeling better and better.”