What makes you think he lacks motivation though? From the last few games that I watched Onana makes too many mistakes and it's doubtful a lack of motivation is the issue. He performed quite well before, saving Manchester United from losing. Judging from interviews he has a strong personality, if that's the right word for it. I don't think motivation is the issue for him tbh, just consistency on stopping goals. It's not a bad choice to look for another first-choice goalkeeper and put Onana as backup IMO. CMIIW.