Messi didn't play but there is guarantee Argentina would have won if he played either. Plus, they got a red card in the first half you know. It was still going to be a tough game for them.
Apart from that, they already secured their lead in the group in the end. They didn't need the win. Maybe they beat Venezuela anyway but Ecuador are much more powerful. They benefited from it well.
The red card was not just the challenge alone that was faced by Argentina. The Ecuador guys actually gave them a lot of pressure from beginning to the end which was what emanated to the red card and subsequently the penalty opportunity. The Ecuador team made good use of long passes to immediately break into the Argentine defence. The really played an impressive game in my submission and deserved what ever favour today's match came with for them. The absence of Messi was indeed one of the major contributory factors to their failure as a match without Messi for the Argentine guys seems like a lost match already although I may be wrong with this but they don't have a very persuasive strikers in my opinion and it is the reason why they just couldn't count goals today.