The 23-year-old proved his fitness by playing his first competitive 90 minutes for more than four months in the Under-21s' 1-1 draw with Newcastle United on Monday afternoon.
McCarthy was named on the Royals bench in last weekend's 1-0 defeat by Manchester United at Old Trafford.
He said: "It was a great feeling to be back on the bench on Saturday, remembering what it's all about and going to amazing grounds.
"It was just a shame about the result, but it's nice to be involved with the first team again. The shoulder feels good, if not stronger, so I've rehabilitated really well.
"Everything feels good - I feel really fit and I'm raring to go."