Marvez reported Jan. 18 there was a "strong possibility" Mangini would return, but once rumors surfaced that Houston Texans linebackers coach Mike Vrabel had been offered the job—which he's since turned down—Mangini's fate seemed sealed.
In Mangini's one season as defensive coordinator, the 49ers were fourth-worst in yards allowed (387.4 per game) and 18th in scoring defense (24.2 points). He spent the previous year as tight ends coach under Jim Harbaugh.
The firing comes at an inopportune time given most of the league positions have been filled.
Kelly has been proactive in bringing together his new staff, as he's expected to hire Derius Swinton as special teams coordinator, per Matt Barrows of the Sacramento Bee, and will hire Detroit Lions running backs coach Curtis Modkins as offensive coordinator, according to ESPN's Adam Schefter.
Kelly did most of his housecleaning Jan. 19, firing 10 assistants with the exception of Mangini. The next day, he indicated he was looking for fresh faces on offense who brought new ideas, per Barrows, and that clearly has extended to his defensive approach as well.
"I'm really looking for diversity, someone that comes from another system so that we can continue to add and make what we do better," Kelly said. "... I really want to bring some people in that have some different viewpoints, have some different ideas and have done it differently."
Kelly is getting a second chance of sorts in San Francisco and clearly wants to ensure his staff is assembled his way. But by bringing in essentially a brand-new crew from top to bottom, the pressure to succeed will be immensely high.