Free-agent center Joakim Noah hasn't been on an NBA roster since he was not re-signed by the Memphis Grizzlies after his one-year veteran's minimum contract expired at the end of last season. Now we know why he hasn't been signed: Achilles surgery.

The NBA veteran announced on his Instagram Wednesday that he has been rehabbing his Achilles injury after having the surgery four months ago. Noah included a video of him working out and mentioned his past with injuries:

https://www.instagram.com/p/B76vq16Bb_Y/?igshid=1tgf5knolr5rk

There had been rumors of an injury, but nothing had been confirmed until now.

A former top-10 pick, Noah was one of the best centers in the league for a period of time in the early 2010s. Spending his first nine seasons with the Chicago Bulls, the 6-foot-11, 230-pound big man was named to two All-Star teams, made the All-NBA Defensive Team three times and was even named the NBA's Defensive Player of the Year for the 2013-14 season.

Noah was an integral part and defensive anchor of the Derrick Rose-led Bulls teams that competed in the Eastern Conference during that time.

Despite his early success in Chicago, injuries have hindered Noah's career in recent years, leaving him fighting for a roster spot and playing time in most cases. A hefty contract from the New York Knicks in 2016 that he didn't come close to living up to also didn't help his cause.

Joakim Noah was actually a free agent for most of 2018 before the Grizzlies signed him in December. In 42 games with the team, the veteran center put up a respectable 7.1 points, 5.7 rebounds and 2.1 assists per game in only 16.5 minutes a contest while shooting 51.6 percent from the field.