McMichael, who can become a restricted free agent on July 1, had 46 points (14 goals, 32 assists) in 78 games this season.
"McMichael's got speed," Armstrong said. "He's got the ability to play multiple positions up front. A little bit younger, we've got control over him for a couple years. The first-round pick was obviously enticing for us entering the draft (Friday), and we're really excited about the Swedish prospect from his draft year last year. He went 37th overall. We love his competitiveness, we love how he can affect the game. There's a growth window there with some players."
McMichael said he is looking forward to joining his new teammates in St. Louis.
"I'm super-excited," he said. "I think one of the first things I did when I found out is checked the Blues roster and got familiar with the guys. From what I saw, there's going to be a lot of years where we can be competitive. It's a team that's looking to build and get better and better each year. There's quite a few guys the same age as me (25), so that's real exciting. I'm already familiar with a few guys as well. I'm really happy to be a Blue.
"Last year was a bit of a down year for me. ... the assists were still there, but the goal scoring (down from 26 two seasons ago), not so much. That's definitely something I pride myself in. I like to get into the dirty areas per se, and find the soft spots and find the back of the net. I think I'm a well-rounded player that's going to be able to score and set guys up."
Selected by the Capitals with the No. 25 pick in the 2019 NHL Draft, McMichael has 154 points (67 goals, 87 assists) in 315 regular-season games, and eight points (five goals, three assists) in 18 playoff games.
“We would also like to sincerely thank Connor for everything he has contributed to our club," Patrick said. "Since being drafted in 2019, Connor has represented our team with professionalism and class both on and off the ice. We wish him and his family nothing but success as he begins this next chapter with St. Louis.”
Gastrin was selected by Washington in the second round (No. 37) of the 2025 NHL Draft. The 19-year-old had 24 points (10 goals, 14 assists) in 39 games for MoDo in HockeyAllsvenskan, Sweden's second-highest professional league, this season.
The Blues (37-33-12), who finished four points behind the Los Angeles Kings for the second wild card into the playoffs from the Western Conference, now have four picks in the first round of the 2026 draft at KeyBank Center in Buffalo beginning Friday (7 p.m. ET; ESPN, ESPN+, SN, TVAS). St. Louis also has picks No. 11, No. 15 (from Detroit Red Wings) and No. 29 (from Colorado Avalanche via New York Islanders).
NHL.com senior writer Tom Gulitti and independent correspondent Lou Korac contributed to this report