The Costa Rican, signed on a season-long loan from Swiss First Division club FC Sion, was guilty of missing two sitters in the recent defeat to Derby.
But Johnson is confident the 27-year-old is adapting to the demands of English football's second tier.
"People must not be too quick to judge Alvaro," he told the Bristol Evening Post. "They have to remember how difficult it is to come out of the Swiss First Division into a league as competitive as the Championship.
"I would not have brought him on board unless I had faith in him and I'm sure he will do well here.
"He is the first to admit that he could have scored more goals, but he already has one and, when he gets another, more will follow. Alvaro is a natural finisher and I'm not worried on that account."