In an interview with the Spanish sports daily As on Friday, the former Real Madrid coach said Torres's poor club form was a worry.
"There are five months until the European Championships during which everything could turn around but if the situation does not change it will be hard for him," Del Bosque said.
The 27-year-old striker, who scored the goal against Germany in 2008 that won the Spanish their second European title, has netted just five times in 23 games since his January move to London from Liverpool and seven of his 12 Chelsea appearances this season have been as a substitute.
"Fernando deserves respect for his record and he has our confidence but if he is not playing, there are certain things that are obvious," Del Bosque added.
"If he plays and he is on form he'll go to the Euros. He has contributed to the successes and he has room for manoeuvre."
Torres's form has become more of an issue for the world champions since the nation's all-time top scorer David Villa broke his leg playing for Barcelona this month.
Villa should return in time for the tournament in Ukraine and Poland but Del Bosque has other options for the forward line, including Athletic Bilbao's Fernando Llorente, who is currently out injured, and Sevilla's Alvaro Negredo.
Valencia captain Roberto Soldado, who has scored 11 goals in La Liga and five in the Champions League this season, is also pushing hard for a recall.
Del Bosque told As that if Soldado maintained his form he would be selected.
The 26-year-old has represented Spain twice, in June 2007, when he failed to score.
"If Roberto Soldado maintains his current form we'll include him in our plans," Del Bosque said.
"In the last few months Soldado has stood out, scoring goals which is the barometer of forwards, while others have been in difficult situations," he added.
"Villa and Llorente are injured, Torres has not played much. What nobody should doubt is that I will try to be fair."
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