Don't forget trainging and MOTM awards you get skill points from that as well so to get an 18 y. o. from 4* to 5* takes less than 25 days! Always try to invest in younger players with high market value (the higher the value the faster a player should train).

But a good strategy for 1st season (my strategy) is to buy 1-2 old 5* players to help you win the league and sell them at the end of the season while the MV is still higher.