I would submit a ticket to see before I took that advise.
A senior can well leave if he does not get the playing time he wants, which he will not get if you leave him off the roster and he gets 0 minutes for the year.
I don't think the act of placing a red shirt on him before the end of the season would guarantee that he would not leave because of lack of minutes played.
Some players do leave because of lack of minutes, others do not.
I would not know what percent chance to put on it, but I have certainly lost JRs and SRs from teams when they don't play. In fact, I count on it when I take over a new team if they have non walk on SIM recruits, because if you cut them then you don't get cash until the following season .. if they leave on their own, you do get cash for recruiting that season.