MLB2

Do I need a MLB2 and if so what formation does it  play in?

You don't need an MLB2 if you run formations with a SubLB instead. However, they will still show up if you don't manually sub them out in formations like 3-4 and 4-3 I think. They also will show up on special teams

You need a MLB2 to have a complete roster to play games otherwise your lineup will say it's incomplete. However, it's not required to have a high overall MLB2, but I would recommend having either a fast, big hitter, or sure tackler at MLB2 that is able to get at least one of enforcer, crusher, deflator, secure tackler, tackler supreme, or lurker for 0 AP. Your MLB2 will be on the field in base packages of 3-4, 4-3, 4-6, 5-2, and goal line.

Another big reason to have abilities like enforcer, crusher, deflator, secure tackler, or tackler supreme is your MLB2 is on the field for kickoffs. Having someone that can hit stick the return man with a high chance at a fumble or the ability to instantly fatigue an important offensive player for your opponent is a major advantage. Sevein131 posted a writeup in September detailing what positions are on the field for your kickoff team.