On April 12, 2014, Knapp made his NHL debut when he entered the Buffalo Sabres' net, in relief of the injured Matt Hackett, with 11:35 remaining in a 4–1 loss to the Boston Bruins. By dressing Knapp for the game, he became the ninth goaltender to dress for the Sabres during the 2013–14 NHL season (sixth to play)[3] and established the NHL record for most goaltenders dressed by one team in a single season.[4] Knapp played the entirety of the Sabres' final game of the season against the New York Islanders, a 4–3 shootout loss.
Knapp was designated a restricted free agent, but was not tendered an offer by the Buffalo Sabers prior to the 2014–15 season.[5] On August 6, 2014, as an unrestricted free agent, he signed a one-year AHL contract with the Lehigh Valley Phantoms.[6][7]
After two seasons within the Lehigh Valley Phantoms organization in which he spent the majority of his tenure with ECHL affiliate, the Reading Royals, Knapp left as a free agent to sign a one-year deal with fellow ECHL club, the Norfolk Admirals, on September 14, 2016.[8] In the 2016–17 season, Knapp appeared in 14 games with the Admirals with just 2 wins before he was released on December 19, 2016.[9]