Valtteri started from third and kept the pair of rapid Ferraris in touch throughout the opening stint of Sunday’s race. But the Finn slipped back to fourth as the long-running Daniel Ricciardo pipped him out of the pits.
Valtteri spent the remainder of the race bottled up between the two Red Bulls, unable to clear Ricciardo, but with Max Verstappen covered behind him.
“Missing out on a first Monaco podium is painful,” said Valtteri. “It's been a long week and we've worked so hard, so it's disappointing to miss out.
“I was stuck in the traffic while Daniel [Ricciardo] was running in free air and ultimately, that cost me a place on the podium.
“It's been a really tough weekend for us and we've just been missing pace. Ferrari were very strong this weekend and for whatever reason, their car seems easier to operate, so we have work to do. We have to learn from this, but there's a long year ahead.
“Hopefully this will be our worst weekend this season. Canada should be a different story.”