After completing his junior season at the University of Minnesota, Fasching turned pro and signed a contract with the Buffalo Sabres on March 21, 2016.

He made his NHL debut five days later and scored on his first shot on goal against the Winnipeg Jets. Fasching and Casey Nelson, who assisted on the goal, became the first two Sabres to each record their first career point on the same goal in their NHL debuts since Alexander Mogilny and Dave Snuggerud did it on Oct. 5, 1989, against the Quebec Nordiques. Fasching got his first NHL assist April 9, 2016, against the New York Islanders.

Selected by the Los Angeles Kings in the fourth round (No. 188) of the 2013 NHL Draft, Fasching was traded with Nicolas Deslauriers to the Sabres for Brayden McNabb, Jonathan Parker and a second-round draft pick in 2014 and 2015.

Fasching was a member of USA Hockey's National Team Development Program from 2011-13 and won a silver medal with the United States at the 2013 World Under-18 Championship, getting six assists in seven games.

Beginning his college career at Minnesota for the 2013-14 season, he had 14 goals and 30 points as the Golden Gophers won the Big Ten title. Fasching was named to the Big Ten All-Freshman Team. He also had two goals and four points in five games for the United States at the 2014 World Junior Championship.

Fasching had 12 goals and 26 points in 38 games as the Golden Gophers defended their Big Ten title in 2014-15. He had a goal and three points in five games for the United States at the 2015 World Junior Championship. In 2015-16, Fasching shared the team lead in goals (20) and was a Second Team All-Big Ten selection as the Golden Gophers won the conference title again.