This is the mother of all Celtic what if's...
If Len Bias was able to play in the 1986-1987 season, I think McHale wouldn't have played on that broken foot, and the Celtics could've won that 1987 title, or maybe not, but McHale would probably have a longer and healthier career. A healthy McHale plus Bias could lead to more Celtic rings from 1988 to 1990. Bias could've also helped to ease things for Bird, and allow Larry to play a couple more years while being healthier, too.
Then, by the time Reggie was able to block Jordan's shot 4 times in a game, Bias would be in his 5th season. He was taller than Jordan, his jumper was better and he had that ferocity Jordan had. Then you add a rising star in Lewis and healthy yet aging McHale and Parish. Only Bird's bad back would stop you from thinking these guys would've crushed any other team until 1993. After that, two super stars in Bias and Lewis, plus a decent supporting cast could still be at the top of the East and the League.
I'd say Celtics could've won another 2 from 1987 to 1990, then a couple more until 1994 (Big 3 retiring). After that, depending on their suporting cast, maybe another one or two. Jordan probably doesn't retire in 1993, and the Bulls would still win 2-3 rings, because they were a great team, but a Celtic team featuring the Big 3 + Bias and Lewis would've been the best team in the 90s.