If we had put him onto the 'A' list now, then we would never have been able to class him as a youth-team player. So we needed to wait for two games to do that."
Had Januzaj been on the A list, the United manager would have had to omit a senior player from his squad.
Januzaj has been linked with Manchester City but when asked whether he expected the youngster to agree a new deal, Moyes replied: "Yes."
United will also be without the suspended Nani for Leverkusen’s game at Old Trafford but could welcome back Shinji Kagawa, who sat out the Palace game.
Moyes added: "Shinji's better although he's still feeling symptoms in his throat. He's had a bit of flu as we said on Saturday. But he came in and did a bit of training yesterday [Sunday] and he's done a little bit today [Monday]. So he's feeling a bit better."
The Scot confirmed that Wayne Rooney will need to wear a headband again if he plays, but another United striker is out of the reckoning after Federico Macheda agreed to join Championship club Doncaster on loan.
The 22-year-old Italian scored on both of his first two league appearances for United in 2009 but has not found the net for them since 2011.