Brian Williams launches his new career with MSNBC on Tuesday — but sources tell Page Six he’s already been causing trouble by calling the shots behind the scenes of the flailing network’s recent shake-up.

“Brian has been going to town at MSNBC,” said a TV insider. “He’s dictating personnel changes.”

Pat Burkey — Williams’ executive producer at NBC’s “Nightly News” who remained when Williams was suspended and Lester Holt got the gig — was relegated this week to executive producer in charge of afternoon news coverage at MSNBC.

“Pat’s going with Brian, but it’s being looked at like a demotion,” said a source. Burkey began working with Williams in 2000 back on MSNBC. Sam Singal has been named the new “Nightly News” EP.

Izzy Povich, MSNBC’s VP of talent and development who previously was a producer for Lawrence O’Donnell, is being shifted to executive producer of MSNBC morning news coverage as part of the shake-up this week.

“Izzy and Pat are a formidable team — and they both have the distinction of having been with MSNBC on our very first day in 1996,” network president Phil Griffin wrote in a memo.

A source sniffed: “Pat and Izzy were at MSNBC before, and they rose up. Now they’re being forced to go back because of Brian.” But an MSNBC rep insisted: “These were two of the most sought-after positions in news. Dozens of people raised their hands, including lots of résumés from CNN, but ultimately we found two outstanding and homegrown talents to help guide MSNBC dayside back to its hard-news roots.”

The moves, among others, are the biggest since Andy Lack returned to NBC News as chairman. A source said: “Andy’s first task was to resolve the Brian issue . . . now his priority is to reinvigorate [MSNBC] daytime.”

Williams returns Tuesday as a “breaking news anchor” on MSNBC after a six-month suspension from NBC. He attended daughter Allison’s wedding on Saturday, Page Six exclusively reported.