Frank Lampard could miss Chelsea's top-of-the-table clash with Manchester United on Sunday after limping out of his side's Champions League defeat to Shakhtar Donetsk after only 18 minutes on Tuesday with a calf injury.
The 34-year-old midfielder was having a scan which will determine whether he can play in the Premier League match at Stamford Bridge, the first of two matches between the sides in four days.
They are also meeting each other at Stamford in the League Cup next Wednesday.
"It's a recurrence of an old injury," Chelsea manager Roberto Di Matteo told reporters after the 2-1 defeat in Ukraine.
Lampard was ruled out of England's World Cup qualifiers earlier this month against San Marino and Poland with a calf problem.
"He was training fine and there were no alarm bells before the game," added Di Matteo. "He was in good condition to play."
Chelsea are four points clear of second-placed United in the Premier League table after winning seven of their first eight matches of the season.