Congress is not in favour in making Dr. Manmohan Singh the next President and Dr. Singh himself is not interested in that position. Mamata should have had minimum courtesy to talk to Dr. Singh and find out whether he is interested in contesting for the post of President.
After all, her party is still with the UPA. Her announcement about him is morally unfair.
Your vote on this answer has already been received
If PM moves to Presidents chair - This is a coalition government, and if congress can have their Prime Minister and President for last so many years, then alliance partner leaders like Sharad Pawar, Mulayam or Alagiri also expect to be offered the highest positions by rotation.
The favourite today appears to be Mulayam who was seen on the dais seated next to Sonia during the celebrations of the 3rd anniversary of UPA.