So this happens in OOTP quite often. You got some guy, he's like a 1 WAR player, but even the AI knows 1 WAR isn't worth much if anything prospect wise so you get rule 5 eligible types -- but I mean at that point you just wait a week and try again.
I dunno, it's a really minor fringe move that even if you nail 20 times in a row hardly matters but you should always try and get a non rule 5 and non 40 man guy back when you are trading these 1 WAR types ,in vidya or real life. Now the Sox will probably be in a position to cut him later in the year (or add to 40) or he's just so bad nobody even wants him in the rule 5 and you've turned a decent backup into a career minor leaguer.