It isn't always about learning a lesson. At times it is about pure talent and the ability to bring it on. Rashford is not among the top three offenders in Barcelona. I doubt he will ever be a starter for more than three games in a row unless he must play because the others are injured. Under normal circumstances he will be coming from the bench or play when Flick knows he can give others a rest. There is just no position for him where he would instantly be considered the best among those who are available, simply because he isn't. He has good moments, like Sancho for example, but they can't play on the same level for very long.
I don't even know the reason why Barcelona choose to sign Marcus Rashford on a loan deal because this player is just clueless, no tactics, no skills. He will always be in the bench if he doesn't make any changes because Hansi Flick doesn't take shit and he likes players who are disciplined and focused on the teams success so it will be a draw back if Marcus Rashford is featured in matches and he doesn't perform as he should. Barcelona should just return him back to Manchester United. Rashford doesn't have a resilient mentality if not he would have been happy to play for Barcelona knowing how prestigious the team is.
If the goal was to have some backup player who would have to accept to be sitting on the bench of the time, maybe then this made sense for Barcelona. They are not paying a loan fee and I think Barcelona pays him around €7 million net per year. If you think about that, it is a good deal. They have a player who is not catastrophically bad, has relatively expectations regarding playing time, and will probably put in a lot of effort when he gets his chances. It is tough to get very strong backup players because they wouldn't upfront accept to be a bench player.