You're both right (above), I think. Sather's narrative for 1986-87 was: (a) improve team defence, and (b) let Gretzky rest if he needs to... except against Calgary, which was now enemy #1.
The Oilers ended up finishing 1st overall that season, but it wasn't easy. Through much of the first 1/3 of the year, they were seen as a disappointment, vying with Winnipeg in the Smythe and struggling to win games on the road. They went 10-8-1 in the first month and a half, and 14-10-1 in the first 25. Not bad, but way off their pace of a year previously. It was the hangover from the loss to Calgary. Not only that, but Coffey, Anderson, and Kurri were all struggling by their standards.
If you've seen the Boys on the Bus documentary, it shows the team coming back to the locker room after a home-ice overtime loss to lowly Chicago. That was game 25, and was sort-of the peak of their early-season blues. They're all pissed, and the room is silent. (After that, they went 21-5-3 in the next 29 games.) Anyway, in the first half of the season, Gretzky very much carried the team. He was superb in the first half, with 40 goals in the first 39 games, and 48 goals in the first 49 games. Around mid-season, Jari Kurri (who apparently got his first tongue-lashing from Sather around this time) started getting hot, meaning Wayne started passing instead of shooting more. Wayne scored only 1 goal in the last eight games of the season, and then only 5 in the 21 playoff games.
But yeah, those last games of the season are odd. Wayne not only got a measly 1 goal and 1 assist in those last five games (and then sat out the last game of the season), he also went -8 in the last four games of the season -- having been +77 in the first 74. I think we can assume he was kind of phoning it in, just resting up for the playoffs. That said, you wonder why he didn't sit out more than 1 game. Gretzky did say publicly (I think, after the season was over) that he was physically tired, for the first time in his career, in the latter half of 1986-87.