Gerhard Berger admits he would be "surprised" if Kimi Raikkonen has the "strength, discipline and ambition to return to the front".
The 32-year-old Raikkonen, who quit Formula One after the 2009 season, will return with Lotus Renault next year following two years in the World Rally Championship. The Finn has already warned that he is "more motivated than ever" and hasn't "lost any speed".
However, former F1 driver Berger says he would not have lured Raikkonen back to F1 and questions whether the 2007 World Champion will be up for the challenge.
"I would say it is 50/50 if he is able to reach the top again," Berger is quoted as saying to Austria's ORF. "I would be surprised if he has the strength, discipline and ambition to return to the front.
"Of course he should not be underestimated and I don't underestimate him - there is no question he is quick and it would be good for F1, so let's hope for it and wish him all the best.
"[However] I would not have taken him. Formula One is so unique. If you don't really live it day and night, constantly perfecting yourself and working on it, it is difficult.
"It's hard, hard work, and whether he has the willingness to do this hard, hard work, we'll see."
