Ok this is a criteria that is often hotly debated not only on this forum but by cricket fans all over the world.
Let me explain this in least common denominator terms to rule out any possibility for misinterpretation or misunderstanding.
Match winning ability has very little to do with whether ultimately you will win the match or not. Yes you can quote me on that.
Let me explain this with an example.
Mohammad Asif I believe is a match winner. What does that mean? He goes an wins every match for Pakistan? No
Considering Pakistan's recent form, I would hardly be surprised if stats showed he has featured in more defeats than wins.
What I mean is, he has the ability to place his team in a position of command and control of the match from where they have a very very good chance of winning the match.
This is an example.
2nd Test: Australia v Pakistan at Sydney, Jan 3-6, 2010 | Cricket Scorecard | Cricinfo.com
He set up the match for Pakistan with excellent seam bowling. He dismantled the mighty Australian top order and helped Pakistan bundle them out for 127.
That is match winning ability.
Were Pakistan in a commanding position from there? Yes
Were Pakistan in complete control of that match from there? Yes
Could Pakistan have won the match from there? Yes.
Did Pakistan win the match? No.
Does that take anything away from Asif's stupendous effort? No
Is he still a match winner? Yes.
It is often beyond the ability of one player to win a test mach completely on his own. Actually, correct that, it is impossible for one player to win a
test match completely on his own.
When I say Sehwag has match winning ability, I mean he has the ability to set up a test match for India, place them in a position of command and control. Whether they ultimately go on to win that match is irrelevant. If they do, kudos to Sehwag and the team. If they dont, shame on the rest of the 10 players.