Forward Adam Cracknell also was recalled from Peoria of the American Hockey League.
Allen has been recalled under emergency conditions due to goaltender Jaroslav Halak’s lower body injury, which he suffered during the first period in the Blues' 4-1 win against the Minnesota Wild on Monday night.
Hillen, 26, has registered one assist and 10 penalty minutes in 10 games with the Capitals this season. Hillen ranks eighth on Washington in average ice time per game (18:21) after missing 25 games with an injury.
The New York Islanders will find themselves in eighth place in the Eastern Conference at the end of Tuesday night with either a victory or a post-regulation loss to the Winnipeg Jets at Nassau Coliseum.
The Islanders begin play Tuesday in ninth, but have 37 points, tied with the eighth-place New York Rangers, who aren't in action again until Wednesday.
The Jets hold down the third seed by virtue of leading the Southeast Division, but they've lost three straight and are four points ahead of the Carolina Hurricanes and Washington Capitals, who face each other Tuesday.
Here's how the lineups could look for the Islanders and Jets:
Mitchell had surgery on his knee in the offseason and reinjured it during the lockout. He had been trying to rehabilitate his leg during the season, but ESPN's report states that Mitchell had arthroscopic surgery on the injured knee and will miss the remainder of the season.
The veteran blueliner had career-bests of five goals and 24 points in 76 games last season, and was a plus-7 in helping the Kings win the first Stanley Cup in franchise history.
This comes on the heels of multiple outlets reporting Monday that the Maple Leafs were granted permission by the Flames to speak to Kiprusoff, who is likely to be dealt before the NHL Trade Deadline passes Wednesday.
I watched many times this year the series between the Russians and Canada in 1972, and he was a dominating player there. After I watched the tapes, I respect him a lot more because he turned the series around. He was the guy. In that time, he was the best in the world. It’s a big honor for me to tie him.
— Panthers forward Jaromir Jagr on scoring the 717th goal of his career to tie Phil Esposito for fifth on the NHL's all-time goals list after a 4-3 win vs. the Lightning on Sunday