The Boston Celtics have told rival teams their core players are not available in any trades, according to Brian Windhorst of ESPN.

The Celtics core is made up of Kemba Walker, Jaylen Brown, Jayson Tatum, Gordon Hayward and Marcus Smart.

The reality is, the Celtics don't have easy options even if they were determined to upgrade in the middle. While one could have some fun with the Trade Machine, Boston has made it clear to anyone who has asked that their core players are absolutely not available. That includes, sources said, both Hayward and Smart, players who have been floated as possible trade chips in the past.

The Celtics lost All-Star big man Al Horford to the Philadelphia 76ers in free agency in the offseason. They also traded Aron Baynes to the Phoenix Suns.

While Enes Kanter is a solid player, the Celtics need to upgrade their center position if they want to seriously contend for a championship. However, Walker, Brown, Tatum, Hayward and Smart aren't going anywhere.

After losing to the Los Angeles Clippers in overtime on Wednesday, the Celtics are now 11-3 on the season. They are tied with the Milwaukee Bucks for first place in the Eastern Conference standings.