For whatever it's worth, in the 3 point era, 19 of the top 20 single season Win Shares totals among Celtics players classified as C, F-C, or C-F belong to McHale and Parish, with the other one belonging to Raef Lafrentz (7.5 WS in 04/05).
The next highest WS season belongs to Al Jefferson in 2006-2007, when he garnered 6.6 WS at age 22.
The top WS season, by McHale, was 14.8 WS in 86-87.
If you include players classified as "F", the top of the list includes many seasons from Larry Bird (peaking at 15.8 WS in 85-86), with a single season of Cedric Maxwell (12.2 WS in 79-80) and a single season of Kevin Garnett (12.9 WS in 07-08).
Last year, Al Horford had 9.4 WS for the Hawks. He's had at least 8.7 WS in 3 of the last 4 years, with the exception being 2013-2014 when he only played 29 games.
So it looks like the Celts can reasonably expect from Horford at least one season that will be the best they've gotten from a traditional big man since at least Kevin Garnett in 2010-2011 (9.0 WS), and the best season from a nominal center since Robert Parish in 1990-1991 (10.0 WS).