He should be, money lost is always a good reminder that we need to look into our strategy and figure something out, so we don't make the same mistake in the future. I'm sure he probably knows that and will or is making a different game plan right now. If not then he will lose his money pretty quickly.
No Warren Buffett doesn't need to worry about his investment in British grocery chain Tesco. His investment company, Berkshire Hathaway, is Tesco's third largest shareholder, owning almost 4% of the company. Tesco is still a solid company that will do well over the long-term, just what Mr. Buffett has always looked for. In fact I would be surprised to see him increase his position since the company's shares are currently trading at depressed prices.
Buffet may have lost money in a grocery chain investment, but he is a major investor and philanthropist who has successfully invested and gained in the past. It is in the nature of investing to take risks, so the fact that he lost money on one investment should not worry him in the slightest.
This guy has tons of money! The stock market is full of risks. Someone with his amount of experience would already be aware of the risks. Warren Buffet would not put all his eggs in one basket. I am sure he has a variety of different things he has put his money into!