I recently went through this exact same thing...essentially Yes Tuft is correct, but other things play factors...the safest way is to scroll over the ML YEARS on the player card..it should now say something like 0.020...
0.172 is a full season and will reflect on the player card as 1 year. So the safest thing to do is find out his number...and add that as the number of games you must keep him down for on top of the 20. Theres other issues too, that may not be a factor in your guess but you cant send a guy down fro 7 games on season 8 the next and 5 the next to achieve the 20, as any games less than 20 on the farm are added to his ML total.