Ireland had the better of territory and possession in the second half but struggled to make it count as the Argentinian defence made a number of excellent tackles to keep them out.
Sexton miscued the conversion and, when Ireland failed to deal with the restart and Ramiro Moyano took the kick, a silly off-the-ball hit from Peter O'Mahony handed Sanchez the chance to put the visitors in front.
Ireland's Joe Schmidt will be a strong contender to replace fellow New Zealander Steve Hansen when the All Blacks coach steps down, says former captain Richie McCaw.
The ability to bring in experience off the bench, like first-five Dan Biggar and wing Liam Williams, gave Wales the impetus they needed in the closing stages, according to Gatland.
"We didn't really have much of a conversation, I learned about it really when he named the squad", he said.
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"Knowing Mario there will be some cunning plan to it, and we'll have to probably decipher it on the run, once we've kicked off on Saturday".
The Argentina match was O'Brien's first game for the national team since last year's 28-19 win over the Pumas in Dublin but the two-time British & Irish Lions tourist could now be set for another stint on the sidelines after his latest setback.
Gatland also praised the decision to play Scotland as their opening November global opponents last weekend, which culminated in a 21-10 victory that gave them momentum going into the clash with Australia.
"Not being in Australia, we didn't get that opportunity".
After a sustained period of pressure from the resulting penalty, Sexton put Bundee Aki through a gap with a lovely flat pass and the Connacht centre was never going to be stopped so close to the line.
"We were probably a wee bit slow out of the blocks but look Argentina are a quality side and it is another win for us with some good stuff and some things we will need to get better for next week", he told RTE Sport's Clare McNamara.
A performance like this next week against the All Blacks will not be good enough and that message will undoubtedly be driven home as Ireland return to their base in Carton House next week.