The 6-foot-10, 260-pound Australian provided solid backup minutes to Andre Drummond last season.
The Celtics, who topped the Eastern Division previous year, have been active in the off-season player market.
Baynes' strength is defensive rebounding, and that is where Boston needs help.
Baynes, 30, has played five National Basketball Association seasons with San Antonio and Detroit. Baynes should solve that problem. Both Morris and Horford's main deficiency is rebounding, which is concerning.
Baynes may not be a big-time shot blocker and he's not the best defensive backup on the market (Dewayne Dedmon would have been flawless, but too expensive), but he's the best big the Celtics could have afforded. Although Dedmon probably fits the roster better, Baynes definitely addresses the need for rebounding. He averaged 4.9 points and 4.4 rebounds in 15.5 minutes per game, but probably from the start will be the most capable rebounder in the Celtics lineup.
The Celtics have still been severely lacking in their frontcourt, though, seeing as bigs like Kelly Olynyk, Amir Johnson and Tyler Zeller had to leave to create enough cap space for Hayward's max salary. According to Forsberg, the deal is worth around $4.3 million as Baynes joins his longtime Pistons teammate - Marcus Morris, in the Celtics uniform.