If you are trying to build a serious Philadelphia upset case, the cleanest version starts and mostly ends with Tyrese Maxey.
Boston is the heavy favorite for a reason. The 76ers enter this as a No. 7 seed, and the burden of proof is on anyone arguing that the series is truly balanced. That is why Maxey matters so much. He is not just Philadelphia's best hope of stealing quarters or making a game weird. He is the one part of the matchup that can plausibly change the emotional temperature of the whole series.
The numbers available here point in the same direction. Maxey is described as the NBA's fifth-leading scorer. He averaged 30 points in four games against Boston this season. In the last meeting, a 114-98 Celtics win on March 2, he still scored 33. None of that proves Philadelphia has the better player or the better team. It does show that Maxey can produce against this opponent even when the overall result goes against him.
The other important detail is experience. Philadelphia is not asking an untested scorer to discover playoff rhythm on the fly. Maxey has 41 postseason games behind him. That does not make him immune to Boston's defense or the pressure of a road opener, but it does make him the most believable bridge between regular-season scoring and playoff-level shot creation.
Joel Embiid's return from an appendectomy is obviously part of the series picture. But if this becomes a genuine first-round problem for Boston instead of a routine one, it will probably look like Maxey setting the pace of the threat. For Philadelphia, that is the narrow path: not a broad team argument, but one star guard forcing the favorite to feel the series.