In the last game of the football season as a freshman, he sustained a potential career-ending back injury, which required surgery (bone fusion) and to be placed in a half body cast for six weeks.

Because his playing opportunities were limited during the season, head coach Joe Kuharich also tried to incorporate him into the games as a halfback, flanker, and punt returner.

In 1966, he still remained as the third-string quarterback, but led the team to 2 December wins that contributed to the Eagles finishing the season with a 9–5 record and a tie for second place in the Eastern Conference of the NFL.

On April 26, 1967, Concannon was traded to the Chicago Bears along with a 1968 fourth round pick (#106-Alan Bush), in exchange for future hall of fame tight end Mike Ditka.

The transaction was intended to fill roster vacancies created by the retirements of Eagles' tight end Pete Retzlaff and Bears' quarterback Rudy Bukich.

In 1967, he was named the starter at quarterback, leading the team to a 7-6-1 record, which would be George Halas' final year as an NFL head coach.

The Bears decided to return to Douglass in the tenth game against the Buffalo Bills, who responded by throwing 4 touchdowns, but suffered a fractured wrist and was lost for the season.

Concannon appeared in 14 games (13 starts), while setting career marks with 385 attempts, 194 completions, 2,130 yards, 16 touchdowns and 18 interceptions.

In 1971, he was limited to just three starts due to a left knee injury he suffered in the second quarter against the Los Angeles Rams.

He was placed on the taxi squad, but broke his right thumb playing paddle-ball at the Cowboys' practice field, before having the chance of joining the active roster.

[9] On October 29, 1975, he was signed as a free agent by the Detroit Lions, to provide depth after quarterbacks Bill Munson and Greg Landry suffered knee injuries against the Houston Oilers.

In March 1981, Concannon was arrested on charges that he delivered a kilogram (2.2 lb) of cocaine to an undercover drug agent in Schaumburg, Illinois, a suburb northwest of Chicago.

[13] He successfully argued entrapment after admitting he was in desperate financial straits, but Cook County Criminal Court Judge Earl Strayhorn admonished Concannon.