For some players, squad numbers are not important. For others, they're everything. Most, I'd guess, fall somewhere in between, where they may have a favorite or an ideal squad number in mind, but won't throw a fit if they can't get it.
For example, Eden Hazard played with 17 for two seasons before switching to the vacant #10 shirt. Previously, the number was willingly given up by Yossi Benayoun so that Juan Mata could have it. New boy Filipe Luis chose a non-traditional full back number in 5 because that was the number worn by his center back idol Mauro Silva. As legend has it, John Terry picked #26 so that he could sit next to Gianfranco Zola in the dressing room. (Of course Lampard sat next to Terry for the last decade, and their numbers certainly aren't consecutive.) But I digress.
Last game with my number 15 http://t.co/mPrjeUuGPG— Mohamed Salah (@22mosalah) July 19, 2014
Judging by the smiley faces, Salah isn't implying that he's leaving the club on transfer to either another club or the Egyptian military. Instead, he's probably teasing a switch to his preferred (?) number 22, which appears in both his Twitter and Instagram usernames. It's also the number he wore at FC Basel and in the colors of the Egypt national football team.
The 22 shirt currently belongs to Willian, who did wear it initially at Shakhtar before switching to #10. But perhaps he's willing to give up the 22 to take over, say, Hazard's old number 17. I guess we'll see.