I'm not sure that's correct, Ive taken this from Whoscored.com, its a breakdown of his season at Stuttgart and his Copa America appearance the other night. He's clearly most productive as a No9.
I looked at the data on transfermarkt. In the Copa America game, he was on the left of a three with Martínez and Messi but did not score so I can see where you are coming from. In addition, he was an striker for most of last season but his overall Stuttgart stats for every season is a strike rate of 29% as striker and of 30% for the other forward positions he has played.