LiAngelo Ball is getting a co-sign from one of the NBA's top talents ... with Jayson Tatum telling TMZ Sports the Summer League standout is long overdue to have a spot in the league!!

The 22-year-old showed out during his time playing in Vegas over the past week -- scoring 9.6 points in 17 minutes per game ... with several incredible highlight plays.

We caught up with the Boston Celtics star outside Avra in Beverly Hills about Gelo's string of performances ... and he thinks Ball did enough to get him on an active roster for 2021.

"I think so," Tatum tells us. "It's only right, he's put in the work. I'm sure he’ll find his way on a team."