Medlycott admitted: "We won there last year in the B&H Cup, but we know that it is going to be difficult because Yorkshire are playing very well this season. But if we perform as we can then it will take something special to beat us."
Ramprakash and Thorpe are not Surrey's only injury worries.
Fast bowler Alex Tudor is rated "50-50" because of a side strain while Martin Bicknell, the county's most experienced paceman, suffered a neck spasm last weekend and missed the Norwich Union League match against Leicestershire.
Essex, meanwhile, could be without prolific Australian batsman Stuart Law as well as Hussain for tomorrow's meeting with Warwickshire at Edgbaston. Law, like Hussain, has a broken finger.
C&G Trophy fourth round Tomorrow's matches
Yorkshire v Surrey (Headingley); Kent v Northants (Canterbury); Lancashire v Sussex (Old Trafford); Warwicks v Essex (Edgbaston); Notts v Leics (Trent Bridge); Somerset v Glamorgan (Taunton); Worcs v Herefordshire (Worcester); Gloucs v Durham (Bristol).