Sir Alex Ferguson's closest rival has provided an unlikely source of support for the under-siege Manchester United boss.Roberto Mancini could leave Ferguson staring at a season almost in ruins on Sunday if his Manchester City side beat United for the second time this season. But the Italian has no truck with anyone thinking the Scot's position is in doubt. Still No 1: Mancini spoke highly of Ferguson'He can stay as United manager for years,' said Mancini. 'I think he can still achieve everything. 'Even if City are No 1, why can't he stay? It is impossible for any team to win all the time. United have won so many trophies over the years, it is impossible for his reputation to be damaged now.'Mancini has problems of his own, with injury doubts over Edin Dzeko and Mario Balotelli while attempts to have Yaya Toure and Kolo Toure available before the Africa Cup of Nations appear to have failed.Mancini concedes his team are still inferior to Ferguson's, even with their lead in the Premier League and a place in this week's Carling Cup semi-finals.