Let's try to figure out why that happened then. Happ was DH'd that day, which I think we can safely assume was a nod to get him some rest. He had played 19 straight games in LF which also meant he had played in 19 of 20 days. Nico was on the IL so Madrigal was at 2B. Hosmer hadn't been DFA'd yet so he, Mancini, and Mervis were all on the roster. That means that holding other starters in place, you have this group for 1B, 3B, and LF: Barnhart, Mervis, Mancini, Hosmer, Wisdom, Morel, Mastrobuoni
Other important context is that a very good LH SP(Framber Valdez) was on the mound for the Astros, which essentially makes the 3 choices choose themselves(Wisdom, Morel, Mancini). Valdez doesn't have extreme splits so could they have snuck another LH in there? Maybe, but imagine what fans would've said about the lineup that included Mastrobuoni against Valdez at the expense of Wisdom or Morel(who ended up combining to go 2/9 with a BB, 2B, and HR, for 3 RBI).
Mancini isn't an option at 3B or LF, so then the realistic choice we're left with then is which one Morel and Wisdom goes to. At a baseline, you'd prefer Wisdom at 3B, it's the more important position and he's better at it. Plus Wisdom had been playing 3B essentially every day for weeks and is the incumbent, so shuttling him to LF for a single game is at best a little odd, at worst damaging to Wisdom's productivity if he prefers consistency in his role. Additionally, Morel hadn't played 3B since he was recalled and hadn't played 3B in over 3 weeks total, while playing OF most every day in Iowa. So yes, he doesn't have copious experience in LF and Houston is a tricky option. The alternative Ross had was 1) continuing to play Happ and risk wearing him down 2) punt on a lineup spot by playing Mastrobuoni over Wisdom or Morel or 3) weaken 3B defense to put Wisdom(who doesn't play much LF either) in that spot. Maybe the best call was choosing one of those! But the 'I can't believe he made this legit WTF decision' framing doesn't seem to hold up.
I'm sure there's other choices that have less backing, and I have my complaints with Ross too. But this is what I mean when I say that people don't interrogate why a given choice *was* made. It's easy to say(doubly so with hindsight) that it was simple and an unforced error to do X, but to understand what the actual motivation was requires embracing that lots of these calls are very much shades of gray that don't have objective right and wrong answers.