Martinez is a good goalie and doesn't take unnecessary risks like Onana. So, if he is willing to come, I will like it despite his age.
Atleast, if Onana sees a serious competition, he will sit up. It is not only Onana, every position in Manchester United squad needs a serious and competitive backup. This is the only way to intill seriousness in the players and make them fight for the position.
Wait a moment, has Manchester United got a striker? Are they planning to continue with Hojlung?
If MU want to take a chance with Dibu then I think it's fine, the striker thing is obvious that they need another one and urgently because if they want a formation where they only have 1 striker as a hooker then that's another thing, but I wouldn't risk it, now MU need to win and win a lot, because they have a huge losing streak.