Denver Broncos quarterback Joe Flacco is expected to miss four to six weeks because of a neck injury, pending reviews from doctors, a source told ESPN's Adam Schefter.

Flacco received a second opinion on the bulging disk in his neck and, a source told Schefter, will not need surgery, pending reviews from Dr. Robert Watkins in Los Angeles and Dr. Jack Kelley in Connecticut.

The Broncos have announced that Flacco will miss Sunday's game against the Cleveland Browns but have not disclosed an official timetable for his return. Initial reports indicated that the former Super Bowl MVP could miss the remainder of the season, depending on the medical evaluations on his neck.

"Everything is still on the table with Joe," coach Vic Fangio said Thursday. "We can leave him on the 53[-man roster] and wait a while, or we could IR him. We'll make that decision here in the next couple days. ... By the end of the week I think we'll have a better understanding, after getting all of the medical advice, going over it, what plan we'll take long term."

Brandon Allen, claimed off waivers on Sept. 1 from the Los Angeles Rams, will start against Cleveland. Fangio said Monday that the team will decide later this week whether Brett Rypien will be signed and elevated to the roster from the practice squad or whether fellow rookie Drew Lock will be activated from injured reserve to be Allen's backup.

During the second half of Sunday's 15-13 loss to the Indianapolis Colts, Flacco spent much of his time between Denver's offensive series getting his neck attended to by the medical staff. After the game, Flacco said he first felt neck pain "a couple weeks ago" and that "I don't know if I got hit on it [Sunday] or not."

Flacco has been sacked 26 times in eight starts this season. Only two quarterbacks have been sacked more often: Cincinnati's Andy Dalton and Tampa Bay's Jameis Winston.

Information from ESPN's Jeff Legwold was used in this report.