That belief is now being challenged as NHL organizations have realized players mature on their own timetables, some taking a bit longer and others needing far less time.
But there is no denying that the American Hockey League plays an important role in the development of all but the most elite defenseman, who pass directly into the NHL.
Here are four intriguing defensemen finding their way in the AHL this season:
Jacob MacDonald, Springfield (Florida Panthers)
MacDonald, 25, has emerged as one of the great finds in the AHL and established himself as a go-to recall option.
Undrafted out of Cornell University, MacDonald played 72 games with Elmira in the ECHL in 2015-16. His play there (17 goals, 20 assists in 72 regular-season games) and with Toledo (seven goals, 19 assists in 36 games) in 2016-17 caught the attention of the New Jersey Devils.
The Devils signed MacDonald (6-foot-0, 204 pounds) to an AHL contract and he continued to impress. Last season with Binghamton, he led all AHL defensemen in goals (20) and points (55), and his 35 assists placed him third among defensemen. He earned a spot on the AHL First All-Star Team.
Crafty with the puck, MacDonald can run a power play. However, with opportunities limited in New Jersey, MacDonald signed a two-year, two-way contract with the Panthers on July 2. It was his first NHL contract.
With Bogdan Kiselevich (broken jaw) injured in training camp, MacDonald won a job with the Panthers to start the season. Just like in the AHL, MacDonald make a quick impact. He scored on his first NHL shot in a 2-1 shootout road loss against the Tampa Bay Lightning on Oct. 6.
After Kiselevich returned, MacDonald headed to Springfield. He joined an experienced group of defensemen after Springfield overhauled its roster in the offseason. He has eight points (three goals, five assists) in seven games, helping Springfield to a 7-1-0-2 start in the Atlantic Division.
Brendan Guhle, Rochester (Buffalo Sabres)
With the Sabres still rebuilding, they need a prospect like the Guhle (6-2, 196) to pan out. They chose him in the second round (No. 51) of the 2015 NHL Draft, a pick acquired from the New York Islanders in the Matt Moulson for Thomas Vanek trade on Oct. 27, 2013.
Rochester (8-3-1-0) leads the North Division, and there are considerable expectations for a deep playoff run. Rather than rushing Guhle, who played 18 NHL games last season, the Sabres sent the 21-year-old to Rochester, where he has six points (two goals, four assists) in 12 games.
Rochester has a deep group of defensemen, featuring several AHL veterans and fellow prospects Lawrence Pilut and Brandon Hickey. That is the sort of defensive depth that can push a young prospect for ice time yet insulate him from having to take on an oversized role at the AHL level.
Oliver Kylington, Stockton (Calgary Flames)
Kylington, 21, has significant professional experience.
He turned pro at age 16 with Farjestad of the Swedish Hockey League in 2013-14. After two SHL seasons, Calgary chose him in the second round (No. 60) of the 2015 draft and sent him to Stockton for the 2015-16 season. He has remained there since, though he has played one NHL game.
That adds up to 247 regular-season games of AHL and SHL experience as Kylington (6-0, 183) starts his sixth pro season.
After 35 points (seven goals, 28 assists) in 62 regular-season games for Stockton in 2017-18, Kylington will be counted on heavily in his fourth AHL season and continues to try to sand down remaining edges to his game.
Jeremy Roy, San Jose (San Jose Sharks)
"Honestly, I just want to play."
That was the simple goal for Roy (6-0, 195) set forth in a story on the San Jose (AHL) website this summer. For Roy, anything that could wrong did go wrong during the past two seasons.
Since the Sharks selected Roy in the second round (No. 31) of the 2015 draft, he has gone through two season-ending knee injuries. In his final Quebec Major Junior Hockey League season in 2016-17, he played 10 games for Blainville-Boisbriand before he tore the anterior cruciate ligament in his left knee.
After a long rehabilitation process, Roy turned pro last season. Twenty games into his rookie AHL season with San Jose, another knee injury halted his development and sent him into another recovery program that extended into the summer.
The Sharks are in a win-now stage and will not be in any hurry to accelerate the development of Roy, 21, as he tries to make up for two lost seasons.
The AHL schedule, which often is lighter in October as teams settle into the season, tested Roy early. San Jose played nine games in 22 days to start the season, and Roy came through unscathed.
Coach Roy Sommer will count on Roy for significant minutes, something that should help him to make up for some of his lost development time. He has a shot that he can put through traffic, and he has eight points (two goals, six assists) in 11 games.