Mourinho, who will be without Ashley Cole for the Champions League Group E match, confirmed that Gary Cahill and David Luiz are in a straight selection battle to partner Terry in the heart of the Chelsea defence.
Cole will be missing however as he continues to recover from his rib injury, suffered against Norwich earlier this month.
"I need to make a decision between David and Gary," said Mourinho, who feels Schalke will present them with a major challenge.
"We lost the first match at home but won the second away. We now play two matches against the strongest team in the group.
"Schalke are missing a couple of players but so are we. Ashley Cole is an important player and he isn't playing."
Mourinho insists it is far too early to start panicking about qualification to the knockout stages of the competition, though, despite an indifferent start to their Group E campaign.
A shock opening defeat to Basel at Stamford Bridge was followed by a 4-0 thrashing of Steaua Bucharest in Romania and they now face a double-header against Schalke, who top the group.
"I don't think it's yet the time to have the calculator and to be counting the number of points you need to qualify, or to be first or second, or to go into the Europa League (as third-placed side)," added Mourinho.
"We play two matches against the strongest opponent in the group, which leaves us the fifth and sixth fixture to play against Basel and Steaua.
"We need points over these two matches (against Schalke). That's obvious. But we're in condition to play without thinking about finishing first or second."
Germany forward Andre Schurrle has recovered from a leg injury in time for the trip. Schurrle scored a hat-trick in Germany's 5-3 World Cup qualifying win over Sweden last week but missed Chelsea's 4-1 Premier League victory over Cardiff on Saturday.
Terry said: "We know about Schalke. They pose a real threat. They look solid defensively. We understand it's a tough game."
Schalke 04 v Chelsea is live on Tuesday evening, Sky Sports 1HD at 7pm