Why does he need to "show something quickly"? Firstly, it's not as if he's taken away meaningful minutes from someone who has proven to contribute. Secondly, there is a ton of time left until the playoffs begin. It's ridiculously to watch him play 15 minutes as a sub and conclude that he therefore can't possibly become a useful option off the bench three months from now.
That's great because I didn't do that. I did say ne needs to show something soon/quickly, but haven't made any conclusions, merely pointed out that given his recent signing, he doesn't have a lot of time to nail down a spot (unlike Rza). I also said he should show something over the next month (after that we have exactly 4 MLS games remaining, the first of which is a six pointer vs Atlanta, not exactly an experimenting ground).
3 months from now is the tail end of the playoffs. We've seen the desperation move of using Veron in a meaningful role for the first time in the playoffs quite recently. It doesn't work well.
Re the beginning of your comment - if anything, I'd be advocating for him to get some starting minutes immediately so he can sink or swim in the short run and show Armas whether he can be immediately impactful, or whether he's more of a long term signing. You are right that 10-20 minutes aren't indicative of full match performances and judging a player on that is unfair.