The Dutch teenager came off the bench at the interval to net the only goal of a horribly dour game in which the Nerazzurri had laboured 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 were strangely subdued in the opening period, with their forward players struggling to impose themselves on the game. Ricky Alvarez was the exception. The Argentine was involved in everything positive the visitors produced in the first half.
It was from his lay-off 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-hand side. In the first instance, Zarate reacted well to Pazzini’s flick-on 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.
Inter’s best chance of the half also came from a delivery from their No.11, Walter Samuel misfiring horribly on the half hour after Brkic had failed to deal with Alvarez’s corner from the left.
In truth, though, Inter, with just one up top in Pazzini, and Stankovic and Esteban Cambiasso providing little impetus from midfield, were struggling horribly to create openings. Siena coped quite comfortably with what they 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 capitalising on some confusion at the heart of the Inter defence.
Calaio threatened again just after the break, stabbing Daniele Mannini’s cross wide as the hosts began to grow in confidence and become even more adventurous.
However, despite their best efforts, they were unable to force a winner. Instead, it was an increasingly laboured 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 Inter level with their hosts on 14 points.