The Boston Celtics have seldom been healthy this season. From Sam Hauser’s intermittent back spasms to Kristaps Porzingis’ missing the opening month of the season as he recovered from surgery.

Boston Celtics guard Jaylen Brown (7) warms up before a game against the Brooklyn Nets at Barclays Center. John Jones-Imagn Images

Joe Mazzulla’s team came into its title defense with a target on their back. Every team in the league wants to secure a win against a roster that dominated the NBA en route to the franchise’s 18th championship.

Without a fully healthy rotation, the Celtics have stuttered at times, specifically between December and mid-January.

On Saturday the Celtics got another injury scare, as Jaylen Brown left the team’s win over the Brooklyn Nets in the third quarter. He was ruled out for the remainder of the game. Brown had missed the Celtics’ win over the Miami Heat on Friday, too.

When you look through Boston’s current roster, you see a string of stars who have suffered from, or are nursing injuries. Jrue Holiday still has a mallet finger issue.

Brown has a knee problem and tweaked his back against Brooklyn. Hauser’s performances remain inconsistent due to his own back problems, and then there’s the Porzingis of it all.

It must be concerning for Celtics fans, that the starting five of Derrick White, Holiday, Brown, Jayson Tatum and Porzingis has played just 654 possessions as a unit this season. To make matters worse, that five-man lineup is losing possessions by -1.4 points per 100.

Boston has 14 regular-season games remaining. After that, the championship push truly begins. Now is the time for the Celtics’ roster to begin getting fully healthy and stringing together a run of games with the core rotation intact.

At full strength, the Celtics are talented and experienced enough to overcome any threat in the league. Unfortunately, it’s looking increasingly unlikely that we’re going to see a full-strength version of the roster.

With competition around the league having caught up, Boston must find a way to get everyone back on the floor and play at a high level.

If not, the Celtics could fall short in their pursuit of back-to-back championships. If that happens, injuries and a lack of fitness will be to blame, nothing more and nothing less.