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.
Cahill said: "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."
The late goal and the injury completed an unhappy end to the game for Cahill as Chelsea missed the chance to open up a four-point cushion at the top of the table.
Albion, however, performed admirably as they played the pressing game new head coach Pepe Mel has been demanding of his players.
It meant Eden Hazard, the toast of the Blues following his hat-trick that sank Newcastle, was barely given space to breathe as he found himself shackled throughout.
Hazard, however, believes the point could be an important one come the end of the season as the title race hots up between Chelsea, Manchester City, Arsenal and Liverpool.
"It was a puzzling game," Hazard told the club's official website.
"At 1-0 I thought we were managing the match well, but we couldn't get the second goal which would have given us protection, and then the game became difficult.
"I tried to play, we tried to play, but it was a very difficult game.
"It was very complicated whenever I had the ball because there were two or three opponents there.
"They had a plan to defend, as was the case when we played West Ham.
"It was a similar situation except at the end West Brom pushed a lot, their crowd got behind them and it became complicated.
"But it's still a point even if many people are disappointed. I think this is a point that will count.
"You cannot win every game and it was important we didn't lose. We'll improve in the future and we will try to win the next game."
- Sports & Recreation
- Branislav Ivanovic
- Barclays Premier League
- Eden Hazard