McHale broke his foot that year and Walton was hurt also....Celts were not nearly as strong as the previous year.
McHale dealt with injuries for both 1985-86 and 1986-87. For 85-86, he only played 68 regular-season games due to an achilles tendon, but played all 18 playoff games (Boston went 15-3 through the playoffs), and led the Celtics in scoring against the Houston Rockets on way to the NBA Championship (25.8 ppg and 8.6 rpg).
In 1986-87, it was the other way around. McHale was healthy and played through most all of the season, playing and starting 77 games. He had easily his best season as a Celtic, averaging a career highs in 26.1 ppg, 9.9 rpg, and 39.7 mpg. Also tied his best FG % of .604. But he broke his navicular bone in his right foot against the Bulls on March 27th, and was advised by the team doctors to immediately get surgery and shut down the season, but ignored their recommendations and played on the injury (he still played 44 minutes in that Bulls game, shooting 10-15 for 21 pts and 7 rebs).
McHale played every game of that playoffs too, starting all but 2 of them, and averaged 21.1 ppg and 9.2 rpg.
The reason why the Celtics lost were a number of issues, some that you touched on. In 1985-86 the Celtics were so strong due to their overall depth. For years their starting unit was strong simply due to their Big Three of Bird, Parrish, and McHale - at times in the past they had strong benches and weak benches.
In 85-86 they had a reserve group they called their "Green Team", that mostly consisted of Bill Walton, Scott Wedman, and Jerry Sichting as the main core, but also had Rick Carlisle, and Sam Vincent too. In early years, the Celtics rotated Tiny Archibald and Gerald Henderson, Chris Ford and ML Carr, Larry Bird and Cedric Maxwell, and Parrish with former Kentucky star Rick Robey. Players got old and retired or moved on to other teams: Robey for Dennis Johnson, Henderson traded for the #1 draft pick that would be used for Len Bias, and Maxwell traded for Walton.
Then Wedman came in for either Bird or McHale, Walton or Greg Kite came in for Parrish, Carlisle came in for Ainge and Sichting came in for DJ. And the offensive pace rarely fell off for the lesser minutes the reserves played. Walton was a former NBA MVP, Sicting and Wedman were former starters for previous teams and efficient long-range shooters.
So what happened in 1986-87?? Yes, there were injuries: McHale was hobbled during the playoffs, but it didn't really impact him heavily in production. Parrish dealt with BOTH ankles sprained. Walton missed all but 10 regular-season games, but started the postseason strong until the Milwaukee series in the Eastern Conference semifinals. He only played in one game of the Detroit Pistons conference finals, and then only 1 minute. By the Finals with the Lakers, he was almost non-existent.
But that wasn't all of it. The Milwaukee series was a brutal one for the Celtics. Danny Ainge suffered a major quadricep bruise, and wore a leg brace/sleeve for the rest of the playoffs, and was hobbled. Wedman only played 6 games of the season due to bone spurs that needed surgery.
So, in summary, most of the team was beaten down by the time of the playoffs in 1986-87. And THEN, came the playoffs:
The first round was easy - a 3-game sweep of the Michael Jordan Bulls. But then a very physical, heated 7-game series against the Bucks, and THEN an even MORE physical, heated 7-game series against the Pistons. And the Celtics had to navigate this with little of their great bench remaining to help them.
By the time the Finals came up, the Lakers were smelling fresh blood in the waters. By comparison, they breezed through their playoffs: a 3-game sweep of the Denver Nuggets, a 4-1 win over the Golden State Warriors, and then a 4-game sweep of the Seattle SuperSonics. They were rested and waiting for the Celtics, watching their games on TV at home.
At times in several of the games of the Finals, it looked like the Celtics were just gassed, and couldn't stop the Lakers' fast-break offense. Couldn't even keep up with them. They were done....