Sandin, was on the ice for Toronto's practice in Seattle on Tuesday afternoon when video showed he was escorted from the session by a member of the Maple Leafs communications staff.
The blueliner was Toronto's first-round selection (29th overall) in the 2018 NHL draft. He signed a two-year, $2.8 million contract extension with the Maple Leafs in September and has posted four goals and 20 points in 52 games this season. Sandin, 22, just returned to the Leafs' lineup Sunday after missing three games with an upper-body injury.
Toronto general manager Kyle Dubas has been active ahead of the trade deadline, acquiring Ryan O'Reilly and Noel Acciari from St. Louis this month and then trading with Chicago on Monday for defenseman Jake McCabe and forward Sam Lafferty.
Adding the veteran McCabe -- who has two years remaining on his contract -- gave Toronto increased depth on the blueline that would only push Sandin -- who averages 17 minutes, 59 seconds per game -- further down. That helped make the Maple Leafs' young talent expendable to a team like Washington, which has been selling off its veterans in favor of acquiring picks and players for the future. The Capitals traded forward Marcus Johansson earlier on Tuesday and sent Dmitry Orlov and Garnet Hathaway to Boston last week.
Not long after the deal with the Capitals, Dubas continued to work the phones for additional draft capital. Ultimately, he acquired a 2024 third-round selection from the New York Islanders for forward Pierre Engvall.
The Maple Leafs' decision to move Engvall is one that also creates salary cap space as they seek to add at a time in which teams throughout the Eastern Conference are strengthening their rosters.
Engvall was one of several pending unrestricted free agents on the Maple Leafs' roster. He carries a $2.25 million cap hit, which was moved to give Toronto more flexibility entering the final days of the trade deadline.
Maple Leafs center Auston Matthews met with reporters after the team practiced in Seattle before the flight to Edmonton, where they play Wednesday. The Maple Leafs came into Tuesday second in the Atlantic Division and have won seven of their past 10 games. They have been one of the more active teams over the past few days.
"When you look at the East, it's pretty stacked all around," Matthews said. "There's a lot of good teams out there. There's a lot of good teams in the East, and each team believes in themselves and they want to give themselves the best chance to go a long way. That's no different than what we're doing."
As for what it means for the Islanders, Engvall gives them a 6-foot-5 bottom-six center who has scored 12 goals and 21 points in 58 games after scoring a career-best 15 goals last season. Engvall represents a potential scoring boost in the bottom six for a team that is fighting for a playoff spot.
The Islanders are in the middle of what has become a chaotic race for the Eastern Conference wild-card spots. They came into Tuesday holding the first of two spots with a two-point lead over the Pittsburgh Penguins. On the whole, there are five teams that entered the day within five points of the Penguins for the final spot.
Sandin fits the bill for Washington. He's already a solid puck-moving defender who does well in sheltered third-pairing minutes. He also has some term for the Capitals to work with over the next couple of seasons.
The trade primarily allowed Toronto to recoup a first-round draft pick, after it moved one in each of the deals with St. Louis and Chicago. The pick they received in the Sandin deal previously belonged to the Boston Bruins.
Gustafsson, 30, is on a one-year, $800,000 deal and could be a player Toronto flips again before the trade deadline passes at 3 p.m. ET Friday.
ESPN NHL reporter Ryan S. Clark contributed to this report.