Torres suffered the problem in Chelsea's 3-1 win over Manchester United after he replaced hat-trick hero Samuel Eto'o.
After the match, Mourinho confirmed that they had now closed the door on any possible departure for Ba, who had been linked with several Premier League rivals.
"Now we lose Fernando for a few weeks with something in his lateral ligament [in his knee] and that is why we have Demba in the squad," said Mourinho.
"We decided not to open the door to him [to leave] because he is a good player and we need him. The next few weeks is Demba and Samuel and I hope they do well for us."
Mourinho hopes Ba will take his chance in the coming weeks.
"It's time for Demba. It's a pity because Nando (Torres) is in a good moment, but it's time for Demba," he said.
"Demba had a fantastic week; (it was) very difficult for me to leave him out of selection because he worked very, very well. Next matches is for Demba or Samuel. Let's go."