if he hasn't complained and you are meeting the promise - or if he complained but you fixed it (meaning you got the i'm happy now message) - then grades won't hurt you. just be careful that when he comes back from grades, you have him in the lineup (assuming he makes grades semester 2).
if you are behind on promises, failing to make grades can be a real problem. i always play it safe in the beginning of the year with promises, trying to not get a complaint, until the first final grades come out early in conf play. that way i won't lose a player over grades. you may still be able to recover - and even though the post season can't hurt you, if your player is happy when the regular season ends - they can help you recover when you are behind. or at least, that was true before 3.0 - there's been some changes, but as far as i know you can still save minutes promises at least. starts promises seem weird, i'm not sure you could save those - never go under 80% of games started, for a promised start player - unless you want them to quit (which is valid in some circumstances - you get half money - you could take a guy with an important per roll or something, and cut bait if its a miss)