The Boston Celtics' loss to the Dallas Mavericks was "deja vu all over again" for Marcus Smart as their underwhelming start to the season continued.

Boston slumped to a 113-104 defeat on Saturday, dropping the Celtics to 10-10 through the first 20 games.

Viewed as favourites in the East going into the campaign, the Celtics are now six games behind the conference-leading Toronto Raptors and have lost eight of their past 12 games.

For Smart, the post-game feeling after letting another contest slip from their grasp is an irritating one.

"Words can't even explain it," Smart said. "It feels like deja vu all over again.

"We keep saying and doing the same thing after every game. It's getting real annoying. I don't even know what to say to you guys at this point."

Asked what they need to do to turn things around, Smart replied: "I don't know. That's the problem. If I knew, we wouldn't be having this discussion, right?

"Once we figure that out, things will start changing. But until then, we're going to continue to get our a** whooped."

Celtics point guard Kyrie Irving accepted there is some credence to the view that Boston are struggling to handle the favourites tag having excelled in years gone by as the upstart underdogs.

"It's definitely difficult, because you're going through some things that you're probably not used to," Irving said.

"I mean that in terms of having that target on your back. Last year, we got away with kind of just winning games and then going on a winning streak and managing throughout the season.

"But this year having expectations, it's something we all have to get used to as a team. Individually, some of us have been through it.

"Some teams are going to come at us, some teams are going to put their forearm in our chests and try us. The way we are responding now, we have to respond better."