Trading a longtime goaltender who lost his starting job could be the first step toward the St. Louis Blues keeping their Stanley Cup-winning captain.

The Blues traded goaltender Jake Allen to the Montreal Canadiens on Wednesday in a move that clears valuable salary cap space to try to re-sign captain and top defenseman Alex Pietrangelo. St. Louis received a third- and a seventh-round pick in this year’s draft and also sent a 2022 seventh-rounder to Montreal.

The key to the deal is getting Allen’s $4.35 million cap hit off the books to potentially keep Pietrangelo in the fold.

That math equation isn’t solved yet. Even after trading Allen, St. Louis has just over $6 million in space under the $81.5 million cap ceiling. General manager Doug Armstrong joked that the equation was “81.5 million divided by 23” for the Blues roster size, but more than that will be required to keep Pietrangelo, who is expected to be among the top players available when free agency opens in October.

One option is putting injured winger on Vladimir Tarasenko on long-term injured reserve when next season begins, deferring a decision until he returns from shoulder surgery. It’s the third operation in 2 1/2 years on Tarasenko’s left shoulder, and he will be re-evaluated in five months.

Pietrangelo, 30, who lifted the Cup in Boston last year after St. Louis won its first title in franchise history, has said he wants to stay with the Blues.

Trading Allen makes it clear Jordan Binnington is the Blues’ starting goalie. After coming out of nowhere in the middle of the 2018-19 season and backstopping them to the Cup, Binnington was only average during the regular season and he struggled in the playoffs.

Allen gets a fresh start as a clear backup to Montreal starter Carey Price. While the Canadiens have wavered year to year on their backup situation, Price playing a league-high 58 games this season was enough to prompt a change.

The 30-year-old Allen didn’t expect to be starting next season in St. Louis.