Is Munmohan Singh a weak Prime Minister?

When one of his ministers (Raja) luted money peoples' like anything, what Munmohan did? He should have given orders to expel from cabinet and arrest him. But Dr. Manmohan did nothing. He just watched what was going on. He is saying that he was a watchman and was untainted with corruption. But his silence has given strength to the corruptionists like Raja, Kalmadi, Ashoke Chawla etc.