Translation:He is a main player on our team.
There is no way anyone can be A main player, only THE main player. The reason is, main has a superlative connotation, there is only one ultimate main player. Another player (not the main one) can be a major player (maybe not in sport, but this is frequently said in business ) , a key player - like others have noted before, or perhaps a standard player.
I wrote "star", but I guess the meaning is "first team". The problem is that in most sports each position has a name, and each position is important, so they're all "main" players unless they sit on the bench. Maybe if he plays in an important position (e.g. pitcher) he's a "main" player?