The Dutch teenager came off the bench at the interval to net the only goal of a horribly dour game in which the Nerazzurri labored for long periods.
Claudio Ranieri’s men had produced a dreadful display at the Franchi Montepaschi Arena but they snatched all three points when their teenage forward, who had come on at the break for the ineffective Mauro Zarate, fired home with just a minute of normal time remaining.
For a side desperate for points, Inter was strangely subdued in the opening period, with its forward players struggling to impose themselves on the game. Ricky Alvarez was the exception. The Argentine was involved in everything positive the visiting side produced in the first half.
It was from his layoff that Dejan Stankovic was able to call Zeljko Brkic into action for the first time, the Serb midfielder stinging the hands of the Siena goalkeeper with a well struck shot from distance 10 minutes in.
Alvarez also created two decent chances for Zarate and Giampaolo Pazzini with inviting crosses from the right side. In the first instance, Zarate reacted well to Pazzini’s flick but could only direct the ball straight at Brkic. Pazzini, meanwhile, was unable to keep his header down when picked out by Alvarez on 25 minutes.
In truth, though, Inter, with just one up top in Pazzini, and Stankovic and Esteban Cambiasso providing little impetus from midfield, was struggling horribly to create openings. Siena coped quite comfortably with what the club had to offer, while at the same time threatening sporadically at the break.
Emanuele Calaio proved a real menace. The Siena striker put Franco Brienza in on goal with a sublime ball over the top only to see the winger drive over, before then dragging a left footed shot wide of the far post right at the end of the half after capitalizing on some confusion at the heart of the Inter defense.
Calaio threatened again just after the break, stabbing Daniele Mannini’s cross wide as the home side began to grow in confidence and become even more adventurous.
However, despite the team's best efforts, it was unable to force a winner. Instead, it was an increasingly labored Inter who made the breakthrough, Castaignos latching on to Thiago Motta’s through ball before firing low past Brkic.
An understandably frustrated Brienza then collected his second yellow for lashing out at the corner flag as Inter held on to claim a victory which draws the club level with its host on 14 points.