"The pain was getting worse and worse by the end of the (first set) tiebreak. I could barely stand up," Murray told a news conference on Tuesday.
"It's better today than it was yesterday, but I still find it hard to walk. It's a little bit frustrating."
The Murray brothers withdrew from their first-round match on Monday after losing the opening set to Poland's Mariusz Fyrstenberg and Marcin Matkowski.
Monte Carlo Masters
Nadal claims first serve the key
22/04/2007 AT 17:17
Murray is set to return home on Wednesday for a scan.
"I guess I'll get a diagnosis from this and I'll know exactly what the problem is and how bad the inflammation is and how long it's going to take to go away," he said.
The 19-year-old Scot, who broke into the top ten of the ATP rankings for the first time in his career this week, was scheduled to face Marc Gicquel in his opener on Tuesday after receiving a first-round bye at Monte Carlo.
Lucky loser Guillermo Garcia Lopez replaces the Briton in the draw and will face Gicquel in the second round.
Spain's Garcia Lopez will receive the prize money for his second-round appearance against Gicquel, but will not earn the ranking points unless he beats the Frenchman.
Murray had said he was recovering nicely from a groin injury that plagued his preparations for the Davis Cup two weeks ago.
Because Murray was eliminated in the first round last year, he will not lose any ATP ranking points.
Remember you can click on the red LIVE button or the link on the right to follow our LIVE scorecentre.
Monte Carlo Masters
Fed: Rafa forces me to improve
22/04/2007 AT 16:39
Monte Carlo Masters
Nadal wins third Monte Carlo title
22/04/2007 AT 12:17