[2] Upon joining the Red Deer Rebels for the 2016–17 season, Alexeyev quickly began playing top minutes for the team.

[6] Alexeyev returned to the Rebels lineup for the 2017–18 season but missed significant time due to personal matters and a shoulder injury.

[8] As a result of another injury, Alexeyev was limited to three post-season games during the WHL playoffs but he tallied two goals and three assists during this time against the Lethbridge Hurricanes.

[9] Alexeyev finished the season with seven goals and 30 assists through 45 games but was wary of his draft position due to his missed time.

[12] Alexeyev drew comparisons to Jonas Brodin and was praised by Craig Button for being a "smart, effective defenceman who moves the puck, plays with a calm and doesn't make things complicated for himself.

[20] Following his major junior season, Alexeyev rehabbed his knee off the ice with the Capitals American Hockey League (AHL) affiliate, the Hershey Bears.

[20] It was originally suggested that Alexeyev could replace Michal Kempný in the Capitals lineup before he suffered an injury during the NHL's Prospect Showcase.

With the following 2020–21 season set to be delayed due to the ongoing pandemic, Alexeyev remained in Russia and was loaned by the Capitals to KHL club, Salavat Yulaev Ufa, on 4 September 2020.