Note that the new wiki contains this information
Some formations are not allowed in Top Eleven and may lead to a very low ball possession, which in turn reduces the chance of victory.
Throughout the whole match you must have:

  • a goalkeeper (blue)
  • at least two defenders (green)
  • at least one striker (red)
  • at least one player on both flanks.

One player on the flank is enough, but three players on the same flank will also lead to low ball possession.
I'm trying to ascertain whether this information is official and/or represents recent changes in the game engine.