But Sheva's Stamford Bridge career is almost certainly over if he fails to make an impact.
"It's about whether a player does what the coach wants or fits in with team philosophy."
Sheva, who scored for Ukraine in the 2-1 midweek defeat to Italy, claimed to have 'no idea' why he was being overlooked.
The former AC Milan star has been snubbed by Mourinho despite recovering from a hernia op that caused him to miss last season's FA Cup final.
Mourinho added: "I'm not sure why he played well last Wednesday. Italy are the world champions - a team where he knew every one of the players.
"He knew the cameras and Italian eyes were on him so it could be a special motivation for him. He worked very hard and played very well.
"Not just because of the goal but he also produced for his team, did his defensive work, and kept intensity in his game for 80 minutes.
"I was very happy with his performance."
Sheva, 31, gets his chance because Didier Drogba has a knee ligament injury, while Claudio Pizarro only returned from international duty with Peru yesterday afternoon.