unless we're talking about rookies playing in severely depressed housing markets like Phoenix, etc, odds are no one is even thinking about buying real estate.
I'd assume any rookie with a head on his shoulders would have NOTHING to do with buying a mouse or mortgage, and would be investing as much of his money made from his entry level contract years.
(1/4 salary to rent/food/vices, 1/4 in high risk stocks, 1/4 in medium risk stocks, 1/4 in low risk stocks.)
Lets say you have 500K in income clear. You can easily live off 100K per year in terms of food, clothing, rent, alcohol, car. Invest the rest of that 400k, and depending on how well you do/ how much of a risk you are willing to take. you could turn that 400k into anything between 600k-1.5million, quite easily.
Considering some guys have to play in New York City or LA, on a 900k salary per year, which works out to maybe 600K in disposable income per year, the only way they could buy a worthwhile house, is to invest that money in the market, and hope your investments have paid off well enough 3 years later to buy a house...
a mortgage really doesn't make sense, unless the player has a strong urge to live in that city, or have a residence in that city (past your time playing for that team)... because unless you are sidney crosby or Alex Ovechkin... really any player can be traded, at any time.
mortgages would be a really silly way of spending your salary. renting, and investing your money before you buy something big and worthwhile, is what I'm sure a lot of players do.