Can anyone explain the NHL salary cap? From what I can read, many Caps fans have no idea. When Eller was out for Thursday game, Caps skated an extra defense man. Why? Was that a salary cap problem? Why else would team have not had an extra forward travelling with the team?
The salary cap is really complex, but the biggest thing with regard to Eller is that he still counted against the salary cap on Thursday despite not playing. A team gets relief if a player is placed on long-term injured reserve, in which case essentially a team is allowed to exceed the cap up to his cap value in order to replace him while he remains on LTIR. (Right now the Caps have three players on LTIR: Michal Kempny, Henrik Lundqvist, and Beck Malenstyn.) Also, when a player is suspended for an on-ice event, his salary still counts towards the cap unless the terms of the suspension say otherwise (so Wilson still counts this time, although I believe his most recent previous suspension was one where the league decided it wouldn’t count due to the suspension's length). If a player is suspended for an off-ice event (e.g., when Nate Schmidt failed a drug test), his salary doesn’t count; I don’t know whether that applies to players in the COVID protocol. Players who are signed with the NHL team but playing in the minor leagues (Hershey Bears) count towards the cap, but the amount depends on whether the player has a "one-way" or "two-way" contract—the former pays the player the same regardless of whether he’s in the minors or the NHL, the latter pays him less if he’s in the minors. I don’t know whether players loaned to foreign teams (Alex Alexeyev, for example, who’s playing in the KHL) also count.
Then this year, because of the pandemic, there is something called the "taxi squad." It's essentially a way to have four to six players (one must be a goalie, unless the team carries three goalies on the active roster) available on short notice without sending them to the minors. A player on the taxi squad still counts towards the salary cap; as above, the amount depends on his contract. The rules on moving a player from the active NHL a roster to the taxi squad are the same as sending a player to the minors—if the player would have to clear waivers to go to the minors, he has to clear waivers to go to the taxi squad. I believe the Caps' taxi squad currently consists of Daniel Carr, Zach Fucale, Philippe Maillet, Brian Pinho, Michael Sgarbossa, and goalie Craig Anderson. The taxi squad players may travel with the NHL team but need not do so; I don’t know what the Caps' practice is.
The Caps are right up against the salary cap. While they get relief for Kempny, Lundqvist, and Malenstyn, they’ve used all of that relief, which means that when Eller missed Thursday's game, they couldn’t bring in a player from the taxi squad to replace him because they didn’t have cap space (and, for that matter, the same would be true if he doesn’t play tomorrow due to injury because he’s not on LTIR). The current roster has 13 forwards (so one extra—normally, that’s Sprong), eight defensemen (so two extra—normally Siegenthaler and TVR), and two goalies (Samsonov and Vanecek). But of the 13 forwards, Wilson is suspended yet still counts against the salary cap. That leaves 12. With Eller out but not eligible for salary cap relief, the only alternative was to play an additional defenseman or just go with a short roster for that game. But since the salary cap hit would be the same either way, there was no reason to go with a short roster—plus, what if someone got hurt during the game?
Edited to add—Long way of saying that your intuition was correct that it’s a salary cap issue BUT the salary cap is not the reason why they don’t have an extra forward. The real reason why they don’t have an extra forward is the Wilson suspension.
(Edited a second time to fix two typos. I don’t know why my iPad inserted "a" both times I typed "NHL team" such that it said "NHL a team," which makes no sense.)