Weigl faces up to four months on the sidelines after being hurt in a challenge with Philipp Max in the first half, ruling him out of Dortmund's upcoming DFB-Pokal final against Eintracht Frankfurt.
The 21-year-old has made 43 appearances in all competitions for Dortmund this season and Tuchel accepted the midfielder will be missed during his recovery.
"I saw him with the ball. I can only tell you how I've seen it live," Tuchel told reporters. "I haven't watched a replay of it on TV yet.
"Then they put pressure on him. He wanted to find a solution, but gets fouled from behind. In his next movement his foot gets somehow jammed in the ground.
"Afterwards he told me he immediately felt that something cracked and that something bad had happened. And that's what it is. The first diagnosis says that he broke his ankle joint. That means a break of several months.
"He is playing every game for us and he is playing in the centre, especially in a game like today's. He is always important.
"[Matthias] Ginter really did a great job although he also got hit pretty hard in the beginning of the game. But he fought through that quite long in the game. Now we'll try to find a solution for the two games, but that's going to be really hard."
Pierre-Emerick Aubameyang's equaliser claimed a point for his side but Hoffenheim's 5-3 win at Werder Bremen saw Julian Nagelsmann's side draw level on points with Dortmund, who remain in third place.
Captain Marcel Schmelzer insisted his team are still in control of the race for automatic Champions League qualification, with one game of the league season remaining, a home fixture against Werder next weekend.
"Augsburg were very solid defensively and they performed well overall," the defender said. "That said, we were still able to create chances. Unfortunately we didn't manage to take them and only have a point to show for it.
"We still have one game left and it's still in our own hands. That means we need to beat Werder Bremen at home. We don't want to go into the play-offs and we have a cup final to think about as well. We want to stay in third spot in the Bundesliga and bring the DFB-Pokal back to Dortmund."