Is this a shame? 5-4-1 is an illegal formation!?

I was playing a game with the top team of the league.
I was using defensive strategy 4-5-1/ 4-2-3-1 (DL DCx2 DR, DMC x2 , ML MC MR and one ST) to contend their attack.
Luckily on first half, I lead by 1-0. But my MC got a yellow card during first half.
So during half time, I substituted my carded MC with one more DC which becomes a 5-4-1 formation.
But it becomes a 20-80 possession as my formation is ILLEGAL? REALLY?
At that time I must change the formation, so I put my "DC" to MC position and set him go defensive, which makes it back a "formal" formation for the remaining time.
But it's already too late and my opponent equalized due to the "shxt" delay of this game for any changes.
Once back to normal possession, we both scored one more and tied at 2-2.

WHY 5-4-1 is illegal in this game?
In real life, MANY TEAMS WILL ALSO PUT 3 DCs during the match which become 5 DFs for defensive strategy.
But in this game it is illegal!?
These really make me feeling sad, COME ON JAMES!

I doubt much that 5-4-1 is an illegal formation. I doubt it this much as you telling the truth. Judging by the fact that you signed in this forum back in August 2013 and this is only your 4th post, I doubt your claim has any purpose but enrage/troll on nordeus forum...

5-4-1 is NOT illegal. I have played it before.....

It is not an illegal formation if it conforms to this:

Throughout the whole match you must have:

a goalkeeper (blue)
at least three defenders (green) (DL/DC/DR)
at least one striker (red)
at least one player on both flanks.
at least four players in the opponents half (ML/MC/MR or AML/AMC/AMR or ST)
at least three players in the midfield section (AML/AMC/AMR and/or ML/MC/MR and/or DML/DMC/DMR)

One player on each flank is enough, but three players on the same flank will also lead to 20% possession.

I bolded what I'm guessing is what you ran into since you mention two formations; if you only have ML MR ST in opp's half it leads to penalty.

The problem was the one Cat mentioned. You need to have at least 4 players in your opponent's half for your formation to be legal (nordeus rules!)

5-4-1 is legal. I have played with the same formation and i had no problems
(DL-DC-DC-DC-DR
ML-MC-MC-MR
ST

The formation you mention is a 52(DMC)2w1.
Only 3 players on oponent half, so ilegal formation.

That's really sad, I am planning for a 3 DMCs defensive formation, now it is impossible ... This game invited mourinho as spokesperson, but unable to use his defensive formation , poor!