The record champion was defeated on Sunday in his own hall in game one of the “Best of Seven” series against defending champion Milwaukee Bucks with 89:101 (46:56).
Theis came from the bank and posted two points and two rebounds in a good four minutes.
Bucks top star Giannis Antetokounmpo came up to 24 points, 13 rebounds and twelve assists.
His teammate Jrue Holiday exceeded the Greeks for one point and became the top scorer of the game.
Game two will also take place in Boston on Wednesday (1,00 a.m. local time).
With Maxi Kleber (Dallas Mavericks), a second German is represented in the NBA playoffs.
The MAVs have to ran against main round leader in the west.