The offense is lined up on the line ball, the coach relizes he only has 10 men on the field, sends #81 into the game, he runs onto the field, behind the QB and gets set on the line.

Is that illegal sub

No.

Not illegal at all. It is never illegal to have 10 on the field. It is only illegal to have 12 or more in certain situations. Just as long as they have 7 on the line they can play with 10 all day.

Yeah but if 81 comes running on the field and lines up as a receiver AFTER the huddle is broken, is that Illegal Sub?

Canadian Philosophy
 

We have something called the gates. When the gates are up, no further substitution is allowed - offensive or defensive. You have to play that play with the players are on the field. For the offense, they may reduce the number of players required to be on the line by the same number of players they're short.

How do the gates come up? The two side guys extend their arms out from their shoulders, much like a school crossing guard, when two or more offensive players have broken the huddle, or after the RFP whistle in a no0huddle offense.

And we play with 12 players, so having 11 is short, 13 is a foul. 13 in the huddle is ok as long as one guy runs off after two or more break the huddle.

schmitty1973, it depends. All offensive players must be momentarily within 15-yards of the ball after the RFP has been given. (see 7.2.1).

Why would this be a problem? There is no rule that prohibits this from happening. If A81 and the rest of team A meet all of the requirements at the time of the snap, A81 coming on to the field late is not a problem.
Dale Smith

81 can come onto the field and participate after the ball is blown ready for play and the other 10 players are in a set position. All 81 has to do is to be one his side of the neutral zone in a legal position (legally on the LOS or legally in the backfield), within 15 yards of the ball at some point, and has been set for at least 1 second. That last one is the hardest one for him as the ball may be snapped before had can get that done.

So if it's 4th and 1, and the K team lines up to punt, a WR can come off the bench while they are already lined up and be a legal reciever? That doesn't sound right.

Yes, but he can't violate any of the restrictions warrenkicker mentions.

Possible IP
 

Remember that there is one further restriction, NF 9-6-4c: <i>It is illegal participation... to use a player, replaced player, or substitute in a substitution or pretended substitution to deceive opponents at or immediately before the snap or free kick.</i>

Yes, they may insert their 11th man as a receiver after the other 10 players are already set, provided he meets the other restrictions. However, if the offense appears to be trying to sneak the player in (especially if this happens more than once), we should shut the play down and toss the flag for IP.

A new NCAA rule this year is even more explicit about this, requiring time for the defense to make counter-substitutions and including a possible delay flag or a USC on the offensive head coach.