Middlesbrough have been left frustrated by midfielder's Nemanja Matic's trial at the Riverside being cut short by an international call-up.
The 20-year-old, who plays in Slovakia with MFK Kosice, has been called up for Serbia's friendly with Poland this weekend, meaning that he has had to leave Boro ahead of time.
Assistant manager Malcolm Crosby told the Evening Gazette: "Good luck to the boy on his call-up, but it makes things difficult for us.
"He looks a good footballer, but he was only here for two days and it's very hard to make a judgement after such a short time.
"We've seen him in his league and he looks very good there, but with respect it's not the best of leagues and we needed to see how he shapes up against Premier League players.
"We've got time to think about it, but it could be at the end of the day that we have bigger priorities elsewhere next month."