Nokia, the world\'s largest mobile-phone maker by volume, said Henry Tirri will take over the duties of Chief Technology Officer Rich Green, who started a leave of absence for personal reasons. Tirri, based in Palo Alto, California, is the head of the Nokia Research Centre, which has offices in a dozen locations from Berkeley to Nairobi. Nokia won\'t provide a timetable for Green\'s return, Doug Dawson, a spokesman for the Espoo, Finland-based company, said by phone yesterday. Green\'s departure came a week after Nokia slashed its sales and profit forecasts, which triggered a slump of the stock to its lowest price in 13 years. Chief Executive Officer Stephen Elop is readying a line of phones based on Microsoft Corp\'s Windows Phone 7 operating system to replace the company\'s own Symbian line, which is losing out to Apple Inc\'s iPhone and Android handsets based on Google Inc\'s system. Nokia targets the fourth quarter for the first Windows handset. Credit rating cut \"The company doesn\'t seem to have much defence for the next 12 months,\" Nomura analyst Richard Windsor said yesterday in at the Open Mobile Summit in London. \"The high-end market is lost and the opportunity for Nokia to come back is in the $200-$300 device market and the Windows Phone for that won\'t be here till next year.\" Nokia\'s long-term credit rating was cut by one level to BBB+ from A- and the company was placed on CreditWatch negative at S&P yesterday. The rating is three steps above speculative grade. It was the second ratings cut for the Finnish handset maker this week after Fitch on June 7 cut its rating on Nokia to the lowest investment grade. Weaker earnings \"Nokia will likely report significantly weaker operating results in 2011 than we had expected,\" S&P analysts including Matthias Raab in Frankfurt wrote in the statement. The \"unexpected and significant sales and profit warning\" caused S&P to reduce its sales estimates to a 10 per cent decline in handset revenue. The rating cut won\'t have a material impact on the company\'s financing costs, Nokia spokesman James Etheridge said in an e-mailed statement. S&P said Nokia\'s \"exceptional\" liquidity position can more than cover its needs for the \"foreseeable\" future. \"We have the assets, we have the innovation,\" Elop said in London yesterday. \"When you go through our labs and resource centres, when you see what\'s available and say oh, if I could just get that faster and working better, we can do very well.\" Green started as chief technology officer in May last year to define \"Nokia\'s technology vision across hardware, software, user experience, cloud services and developer programs,\" according to the company\'s website. He joined Nokia after a 19-year career at Sun Microsystems Inc, where he was in charge of software as executive vice-president.