Both Chelsea and Liverpool will play mostly squad players anyway. They'll suffer if they have many injuries and covid cases. But if they don't have many in January, then it won't be a problem for them: they'll play squad players and if they reach the final then great, if they fail to reach the final, then great as well, for otherwise the fixture congestion in March will be very tough.
Generally, giving playing time to squad/fringe players, especially in cup semis, is not a bad thing per se, for some players (e.g. Keita, Firmino, Minamino...) may hit form and be useful for the league games. It's a bad thing if the resources are depleted, and it's early to say whether they will suffer from covid/injuries in 2-3 weeks. There are positives and negatives from playing those cup games, time will tell which will prevail.
Generally, giving playing time to squad/fringe players, especially in cup semis, is not a bad thing per se, for some players (e.g. Keita, Firmino, Minamino...) may hit form and be useful for the league games. It's a bad thing if the resources are depleted, and it's early to say whether they will suffer from covid/injuries in 2-3 weeks. There are positives and negatives from playing those cup games, time will tell which will prevail.