The Government Center garage, where HYM plans to construct six buildings, eliminating 1,200 of the current 2,300 parking spaces.

The MBTA’s decision to shut down its Green Line subway north of downtown Boston will give the developer behind the troubled Bulfinch Crossing project time to finish demolishing part of a huge garage that hangs over the train’s tunnel, the agency’s leader said Friday.

The shutdown is primarily aimed at fixing elements of the already-open Green Line extension to Union Square, like the overhead power supply for trains, and preparing for the opening of the next extension to Medford, the T said in a press release. All Green Line trains that ordinarily travel beyond Park Street will terminate at Government Center station from Aug. 22 to Sept. 18. The shutdown will coincide with the MBTA’s plans to close its Orange Line subway for a similar period of time in order to repair tracks and replace signals after Federal Transit Administration safety inspectors found a raft of safety issues in need of repair.

“Obviously we don’t like to be in the position of pulling service back like this,” MBTA General Manager Steve Poftak said Friday afternoon. “But in this case, it advances a number of our capital, reliability and safety projects. It also helps get us in position to get the Medford Square Branch open as well.”

The Green Line Extension’s Medford Branch was to open late this summer, but that has now been pushed off until late November.

“The start date has also been affected by the availability of Safety and Operational support crews that were previously prioritized for GLX, but are now re-allocated to other critical MBTA construction work, including in the MBTA’s response to the Federal Transit Administration’s Safety Management Inspection directives,” the MBTA said in a press release.

Poftak told reporters Friday that the T was already planning for a multi-week shutdown of the Union Branch around this time period and that it “just happened to overlap” with the Orange Line shutdown.

“And then there is again as a secondary factor, it is necessary to close the Orange and Green Line at least at Haymarket to allow continued demolition of the Government Center Garage,” he said. “So these overlapped on the calendar in such a way that they we thought rather than having multiple diversions, multiple closures and multiple service modes on this part of the Green Line, we would have it be consistent and we would give our forces the unencumbered access to the entire workspace that they need.”

The garage, site of HYM Investment Group’s Bulfinch Crossing project, is being torn down to be replaced with a life sciences tower and new housing. However, the demolition process has already killed one construction worker when a floor collapsed beneath him in March, sending many tons of concrete and rebar falling nine stories on top of the Green Line and Orange Line tunnels on the site. And earlier this summer, columns holding up the remainder of the garage were found to be dangerously corroded.

Though he said that ridership on the Union Branch has been “pretty good” since the extension opened this spring, Poftak did not have an estimate of the number of people who will be affected by the decision to suspend service.

A T spokesperson later estimated that about 2,500 people board trolleys at Union and Lechmere stations each weekday.

The T’s general manager maintained Friday that while the Orange and Green Line shutdowns will be disruptive, they are responding to calls from T riders who are frustrated with the MBTA and its shoddy service.

“I get that. I’m a regular commuter, I talk to folks on the T. I know that there’s a sense of frustration. And I’ve heard and felt that folks wanted bold and decisive action and what we are doing is bold and decisive,” Poftak said. He added, “I’m in the position of asking folks to be patient and to allow us to make some of these bold and decisive decisions rather than stringing out some of these construction projects, literally for years. We’re going to be able to get them done and get them get them done efficiently, and what you’re gonna have is a better customer experience at the end of it.”