Mourinho had hoped Terry would be fit for Tuesday's Barclays Premier League game at West Brom after sitting out Saturday's 3-0 victory over Newcastle at Stamford Bridge with a double muscle injury.
But the 33-year-old was again missing, with his absence felt as the Baggies staged a late second-half rally to grab a 1-1 draw following Branislav Ivanovic's opener in first-half injury-time.
Mourinho will be hoping the additional game off will have allowed Terry sufficient time to recover ahead of the fifth-round tie with City.
Terry's presence is crucial as Cahill limped away from The Hawthorns with a damaged calf sustained shortly prior to Victor Anichebe heading home an 87th-minute equaliser, meaning he is now a major doubt for the trip to the Etihad Stadium.
Gary Cahill: "Once I got the knock, I was struggling from then on.
"It happened when a West Brom player went for a header and I was on the floor, and he landed just inside my knee/calf.
"It's sore and we'll see what it's like later."
Wednesday February 12
19:45 Arsenal v Man Utd
19:45 Everton v Crystal Palace
19:45 Manchester City v Sunderland
19:45 Newcastle United v Tottenham
19:45 Stoke City v Swansea City
20:00 Fulham v Liverpool
Premier League
1 Chelsea 57
2 Arsenal 55
3 Man City 54
4 Liverpool 50
5 Tottenham 47
6 Everton 45
7 Man Utd 41
8 Southampton 39
9 Newcastle 37
10 West Ham 28
11 A Villa 28
12 Swansea 27
13 Hull City 27
14 Stoke City 26
15 C Palace 26
16 Norwich 25
17 West Brom 24
18 Sunderland 24
19 Cardiff 22
20 Fulham 20