The Boston Celtics may have seen the last of Al Horford in Beantown green, but they haven't lost out on other reliable and significantly more economical big man options. And according to Sports Illustrated's Chris Mannix, among those is Robin Lopez, who was last seen wearing a Chicago Bulls jersey.

As Mannix noted, Boston may offer the $4.6 million room exception to Lopez, but that could be easily dwarfed by bigger offers elsewhere. Lopez is 31 years old and is coming off a payday that netted him $14 million on the final year of his four-year deal signed in 2015.

In 74 games for Chicago last season, the Stanford product averaged 9.5 points, 3.9 rebounds, and 1.1 blocks in 21 minutes per game.

Boston will need to find a replacement big man for Horford and Lopez, while he isn't and will never be the offensive and defensive force that is Horford, could prove to be a solid rotation piece for Brad Stevens.

The Celtics are widely expected to welcome All-Star point guard Kemba Walker to their fold when free agency opens on Sunday.

The team only has seven players under contract with Robert Williams as the lone legitimate option in the middle. Of course, that's not going to cut it if Boston wants a fighting chance to go deep in the Eastern Conference playoffs, making the center position one of the top priorities for Danny Ainge and co. in free agency.