I'd say reject their transfer requests. If someone comes in with an offer equal to their release clause, then thats tough luck. But dont transfer list and maybe you'll get lucky? They'll become happy again eventually if you manage to hold onto them.
As for replacements, instead of Martins, look at Rodrigo Palacio. Pretty much similar players, except Palacio is a bit better
imo (and older as well). Instead of Michael Owen, get Javier Saviola. He'll be a backup at Madrid in all probability and should be available real cheap (7-8M). Similar player to Owen, although maybe not as good. But great none-the-less. These two will ensure that you get two quality players to replace the quality ones going out. You could also have a look at Huntelaar who wont be very expensive either if he's still at Ajax. Have a look at Arturo Lupoli as well. Eidur Gudjohnsen could serve you well too if you need a backup. Hernan Crespo for a year or two would be good.
Also, what teams got relegated from the league in your first season? Have a look see if they have some good/ decent striker and you can probably pick him up cheap. In my many games, I've had West Ham, Everton and Blackburn all get relegated (not all in the same ofcourse!). So players like Ashton, Johnson, Santa Cruz, McCarthy, Yakubu, Bellamy etc were all available cheap.
Owen and Martins 
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