Butt suffered the injury just after the walk-through in the special teams portion of practice, which features full running but no major contact.
It is the third torn ACL Butt has suffered in the past four years but the first to his left knee. He tore his right ACL during conditioning drills at Michigan in the spring of 2014 and then tore his right ACL again in his final game at Michigan in the 2016 Orange Bowl.
"Jake has worked his tail off to recover from his previous injuries, and we feel terrible for him," Broncos coach Vance Joseph said as the team confirmed his latest setback. "He's a great kid who's developed into a very good football player. We have no doubt Jake will come back stronger than ever."
Joseph said after Thursday's practice that Butt was sent for a magnetic resonance imaging (MRI) exam after suffering a left knee injury in practice -- "he's being evaluated for a knee, left knee,'' Joseph said.
Butt was examined initially by the team's medical staff at the Broncos' complex, and at that time there was concern Butt had torn his ACL. Butt was then taken for an MRI, which confirmed the initial exam.
The Broncos had hoped the second-year tight end could become an impact player in their passing game. In three games he had eight receptions for 85 yards. Butt played 36 plays in the season opener against Seattle, 26 snaps in Week 2 against Oakland and 35 plays this past Sunday against Baltimore.
Butt was a fifth-round pick in the 2017 draft by the Broncos. He spent his rookie season on injured reserve as he recovered from his Orange Bowl injury.
Early on in training camp this summer, Butt said, "This is probably the best I've felt in my career getting a whole year under my belt with professional strength coaches.''
"When you've been away from football like I have a couple times, you know you can't take any opportunities you have to play for granted," he added.
The Broncos have two other tight ends on their roster -- Jeff Heuerman and Matt LaCosse -- to go with Brian Parker on the practice squad. Parker would be the quickest move the team could make because he was with the Broncos throughout the offseason program and training camp.