The Boston Celtics plan to sign free-agent center Aron Baynes to a one-year, $4.3 million contract, according to a league source. The 6-foot-10 Baynes, born in New Zealand but an Australia national, averaged 4.9 points and 4.4 rebounds over 15.5 minutes per game last season in Detroit. Baynes provides much-needed help on the defensive glass, grabbing 21.6 percent of all defensive caroms when he was on the floor last season. The Celtics can't officially sign Baynes until a sequence of moves that will include first signing Gordon Hayward. Boston traded Avery Bradley to Detroit last week to generate the necessary cap space to sign Hayward to a maximum-contract salary that starts at $29.7 million for the 2017-18 season. The Vertical first reported Baynes' signing. The Celtics were thin up front with the recent departures of Kelly Olynyk (four years, $50 million with Miami) and Amir Johnson (one year, $11 million with ...