The Lazio striker finished second in the race for the Golden Boot in Italy last season, but he feels the arrival of the Portuguese may have dented his chances of repeating that feat.
"When Ronaldo was confirmed at Juventus, I thought: 'Thank God I had time to win the silver boot last year," he told Abola.
"It will be very difficult to face him, both in the field and in the race for the best scorer. He won the prize many times and will try to repeat in Italy."
Immobile scored 41 times for Lazio in all competitions last season, finishing behind Mauro Icardi in the hunt for Serie A's top award.