Wayne Rooney was sidelined for Manchester United's FA Cup quarter-final replay at Chelsea on Monday after suffering a groin injury on England duty.
When Rooney missed United's 1-0 win at Sunderland on Saturday it was seen as an apparent bid to keep him fresh for Monday's game, but manager
Alex Ferguson, speaking just before kick-off against Chelsea, confirmed a groin problem sustained in the 1-1 draw against Montenegro was the cause of the England striker's absence.
"He (Rooney) got a groin injury playing for England," Ferguson told ITV. "He wasn't 100 percent yesterday (Sunday), so there was no point risking it.
"If you exacerbate it you can be out for weeks. Hopefully he will be back for Monday (for the derby against Manchester City)."
Rooney's regular strike partner, Robin van Persie, was named on the bench, with Ferguson electing to pair Danny Welbeck with Javier Hernandez in attack.
"I've tried to freshen up the squad as best we can. Playing at Saturday lunch-time and then again today requires freshness," Ferguson added.
Chelsea interim manager Rafael Benitez made six changes to the side beaten 3-1 at Southampton on Saturday, with club captain John Terry dropping to the bench.
David Luiz came in for Terry at centre-back, with Ashley Cole, Ramires, Eden Hazard, Juan Mata and Demba Ba also returning to the starting lineup.
Chelsea and United, who drew 2-2 at Old Trafford on March 10, are competing for the right to face Manchester City in the semi-finals.