I feel like Rafinha returning makes Rakitic obsolete, not Cesc. Knowing Lucho's focus on youth, I see Rafinha getting a lot of appearances and resting both Xavi and Iniesta, contributing with his higher energy and versatility to cover for a variety of roles. In my opinion, he's not yet starting calibre in the top teams, but he will get a lot of time against mid-table and small sides. However, the downgrade between him and Iniesta/Xavi is pretty significant, he has high potential but isn't there yet.
You could have said pretty much all of that about Rakitic. Most people attribute him a role of one who offers workrate, running, and the ability to keep it simple in the midfield and allow Iniesta/Xavi to focus on their work. I feel like Rafinha could cover that role just as well, if not better due to his familiarity with Barca's system. There was also a quote floating around about Lucho not wanting to get rid of Roberto, and I could visualize a similar role for him as well, with him tending to rest Xavi and Rafinha tending to rest Iniesta. However, what we lose is starter-level quality. Cesc is a brilliant midfielder who could come in and play against the best teams, and will prove that in the EPL next season without a doubt. He didn't give amazing performances this season against big teams, but he was crammed into a 4-4-2 and used as an extra body to give more freedom to our creative players. If he was incorporated in a 3-man rotation with Xavi and Iniesta, and the best two were to start big games, we would have the quality in case of injury and the depth provided by Rafinha and Roberto.