Sehwag may not have batted much in the middle order in Test matches, but one has to remember that he scored his first hundred (105) while batting in the middle order against South Africa in his debut. If the selectors take this decision to bat him in the middle order, it would also mean that Tendulkar would not be under pressure. Tendulkar with the knowledge of Sehwag batting at number 5 or 6 can play a free-flowing innings.
Let's take a situation, where Sehwag opens the innings, and India are tottering at 40 for 3, which includes the wicket of Sehwag, Gambhir and Tendulkar. The opposition would be suddenly high on confidence and may set an aggressive field for the rest of the batsmen, as they would be new to international cricket. Hence, the new boys would be under pressure with close-in fielders and slip fielders waiting for a catch down their throat. They have a greater chance of crumbling. But, in the same scenario (40/3), lets say Sehwag walks into bat at no 5, the opposition would be on the back foot, as he can change the game on its head with his batting. This would make the other players coming in, to be confident to face the opposition. Even if wickets start to fall, he can play his aggressive game and score as much runs as possible batting along with the tail. One of the best thing about Sehwag coming in the middle order is that the player would be facing slow bowlers, a situation and prospect in which he thrives.
His experience of 95 Test matches for India has made him an intelligent player. He has a good cricketing brain. His 8178 Test runs is testimonial to this fact, else it would not be possible to score such runs with an average of 50.79. He reads the game very well, so he would know how best to attack or defend when he is in the middle. It is this trait of his, which has helped him score so many runs, though there have been many instances, when he has been ridiculed for his batting approach.
But, just his presence on the pitch would help the younger players coming into bat. He will play an important role in keeping the youngsters' nerves down. Some may say that the best players of any team need to bat at either 1, 2 or 3, but it is not necessarily true if it is done in the greater interest of the team, as will be the case with the Indian team, if Sehwag bats in the middle order.