Uruguay's Forlan was seen to be involved in an exchange with Ranieri and never entered the field.
"He did not refuse to come on, I was talking to him about tactics," Ranieri, whose side are joint-sixth in Serie A, told reporters.
Manchester City striker Carlos Tevez refused to warm up while a substitute in a Champions League game at Bayern Munich in September and has only now returned to the squad after apologising.
Ranieri was later quoted as saying that Forlan, who has struggled for form in a mediocre Inter team since joining from Atletico Madrid in August, had not wished to come on if he was not a central striker.
"I asked him 'do you want to play as a winger?'. He said 'no coach'. Forlan said: 'If I come on I would prefer to play in another role. And I said 'OK, thanks'. I appreciate his honesty and fairness," Ranieri mused.
The under-pressure coach, who saw his team crash out of the Champions League in midweek, also denied president Massimo Moratti had stormed out of the San Siro at half-time after Diego Milito missed a penalty.
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