Hibs' Porteous in Tynecastle flashpoint as Scotland see red in under-21s draw
The Hibs defender couldn’t keep out the Croats in the first half and Nikola Moro and Kristijan Bistrovic fired Igor Biscan's visitors ahead.
But he was involved in one of the second-half flashpoints as Sosa was sent off shortly after former Easter Road loanee Glenn Middleton reduced the deficit.
Aberdeen’s Connor McLennan then grabbed the all-important equaliser but there was no late charge from the home side as former Rangers midfielder Billy Gilmour – now at Chelsea – was dismissed just moments after climbing off the bench.
The result leaves Scotland top of Group 4 courtesy of their superior head-to-head record with the Czech Republic who are also on 18 points.
The Czechs face Greece on Friday while Croatia - who sit a point behind - round off their campaign against Lithuania, meaning Scotland can clinch automatic qualification with a win in Athens on Tuesday.
It was an ugly first half as both teams opted to shell the ball long and Scotland found themselves behind 20 minutes in.
Ryan Porteous could only get a slight touch to Lovro Majer's cross, sending the ball to Moro, who cut inside and scored at the near post.
A bad start only became worse five minutes later as Daniel Harvie was caught ball watching and Bistrovic unleashed an unstoppable 20-yard drive into the top corner.
Nathan Patterson was replaced by his Rangers team-mate Middleton at half-time and in just eight minutes Scot Gemmill’s change paid dividends as the winger fired a low left-foot drive which Croatia goalkeeper Adrian Semper let squirm through his grasp and over the line. The blunder was just the boost Scotland needed and they were given another moments later when Sosa was sent-off for a ridiculous kick-out at Porteous' face.
Gemmill hoped the introduction of Chelsea wonderkid Gilmour and Aberdeen's McLennan would turn the game back in his team's favour. It looked to be going to plan as McLennan slotted home Harvie's cross to level with 20 minutes left.
But Gilmour got his marching orders barely a minute later after aiming a verbal blast at Italian referee Michael Fabbri.