I love McHale and Parish, but you just have to go with Magic. And i think that Magic arrival would not be the end of Tiny in Boston. Lakers used in early years Magic with Nixon, so i think Celtics lineup in 1980/81 would look like:
NATE ARCHIBALD
MAGIC JOHNSON
LARRY BIRD
CEDRIC MAXWELL
DAVE COWENS (i believe without Parish in town, he wouldn't retired).
The bench would be:
GERALD HENDERSON
CHRIS FORD
ML CARR
RICKY BROWN (or some other big drafted #13)
RICKY ROBEY.
Our biggest rivals were Sixers back then, and you remember what Magic did to them in 1980 g6 (42+15+7).
I believe we would still win the title in 1981. Win it all in 1982, and later maybe trade Archibald, Brown, Robey with some picks for some bigs (maybe Parish)and we could end up with 5-6 championchips at least in 8 years.
We would still draft Ainge, and Red would make some magic (ala Robey for DJ, Henderson for 2nd pick (Bias)). Every free agent would kill for oportunity to play with Bird and Magic...