The visitors slipped two goals behind at the Hawthorns thanks to a first-half double from Shane Long, but Karim El Ahmadi and Ashley Westwood struck back after the interval to rescue a draw in the west Midlands derby.
Lambert was proud of the renewed energy shown by his charges in the second 45 minutes.
"I can't change the rules, I can't start the match with 12, 13 or 14 players," Mourinho told reporters. "Juan deserves to play.
"'I have to leave players out because I can't start with 15 players. It's a good problem because I have so many good options."
Mourinho also confirmed Fernando Torres is ready to return when Chelsea take on Basel on Tuesday, having missed the Blues' last three matches through injury.
"Torres is ready to play. He trained with the team yesterday and this morning with no limitation and he is ready to play," he added.
Chelsea can secure a place in the Champions League knockout stages with victory on Tuesday, but Mourinho is adamant it is Basel who should be feeling the pressure.
"Basel are a good team. They won in London because they played better than us (but) we are in a better situation now," he continued.
"If somebody tomorrow has to feel the pressure it's not us, it's Basel.
"Tomorrow we can finish the job to qualify. After the first match, we did our job."
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