Following Boston's 109-102 loss to the Brooklyn Nets on Monday, Tatum told reporters that Irving is "telling his truth" and "knows what it takes to win a championship":
Irving noted the team has "a lot of learning to do" and added that Boston needs to embrace its depth:
"I know from the majority of fact that we're better than most teams in this league. It's just going out and proving it every single night and demanding it and actually showing it. So, until we do that every single night and realize our depth is a positive and all the wishes and could-haves and should have done that, once that goes out the window, then we'll be better.
"But until then, we're going to keep having these ups and downs and these lulls of going against teams on the road, and they just know they can take advantage of us down the stretch or when this group is or that group is out."
Before Monday's game, Irving explained the reasoning behind his comments: "I never want to come from a place where I don't want to sound like, or make it feel like, I don't want to win a championship. Sometimes I may come off and say things, never to question my teammates in public like that ever again; I just want to win so bad."
Following Monday's loss to Brooklyn, guard Terry Rozier suggested players aren't used to being surrounded by so much other talent and being forced into unfamiliar roles, according to Vincent Goodwill of Yahoo Sports: "I don't think we've all been on a team like this. Young guys who can play, guys who did things in their career, the group that was together last year, then you bring Kyrie and [Gordon] Hayward back, it's a lot with it."
On the surface, the Celtics seemingly have a great mix with veterans like Irving, Hayward and Al Horford surrounded by younger players like Tatum, Rozier and Jaylen Brown.
The Celtics went to the Eastern Conference Finals last season, but Irving and Hayward were both injured, which allowed Rozier and Brown to take on bigger roles.
Boston has struggled to find the proper synergy this season and is on a three-game losing streak.
The Celtics entered 2018-19 as the favorites to represent the Eastern Conference in the NBA Finals, but they are fifth in the East at 25-18.
If the playoffs were to start today, Boston would have a difficult first-round matchup against the Philadelphia 76ers.
Boston's next chance to break out of its funk will come Wednesday when it hosts the Eastern Conference-leading Toronto Raptors.