LONDON, Feb 17 (Reuters) - FA Cup holders Chelsea could face Premier League leaders Manchester United at Old Trafford in the quarter-finals if both win fifth-round ties. United must beat Reading in their last-16 meeting on Monday while Chelsea, who crushed Brentford 4-0 in a fourth-round replay on Sunday, have to get past Championship (second division) Middlesbrough who they meet at the Riverside on Feb. 27.