Westley, whose name has often been mentioned in dispatches in recent seasons, has enjoyed a timely prolific run with three centuries in his last six first-class innings, the most recent being an unbeaten 106 for England Lions against the South Africans.
That hundred continued a notable record of making runs against touring sides: last season he scored 108 against the Sri Lankans to follow a 99 against the same opposition in 2011, while in 2015 he scored 144 against an Australia attack including Mitchell Starc, Josh Hazlewood, Peter Siddle and Nathan Lyon.
"I'm desperate to play for England, it would an absolute honour," Westley told The Independent, prior to his call-up. "I've felt good the last few years at Essex. My first-class record has been very good. I've scored runs for the Lions and scored runs against the touring first-class teams - I've been in a good space for a number of years. So if there's ever a time to play it would be now.
"I'd like to think facing international attacks brings the best out of me," he added. "Getting the hundred against Australia and backing it up against Sri Lanka and then South Africa, I'd like to think it's not just a fluke.
"Whether I raise my game or whether my game is just suited to that I don't know until I'm exposed to it at Test level. But I can just take a lot of confidence from the fact I've scored those runs against some good bowlers and good attacks. It puts me in quite a good place."
Westley has been named alongside another new name in the Test squad in Middlesex's Dawid Malan, who could yet feature if England decide to included an extra batsman, probably at the expense of the spinner Liam Dawson, when they confirm their starting line-up next week.
Malan made his England debut last month in the deciding T20 against South Africa in Cardiff, where he impressed with 78 off 44 balls. He has been a consistent performer with Middlesex in red-ball cricket, averaging 42.50 in the Specsavers County Championship this season with a highest score of 115.
The notable omission from the squad is Surrey's Mark Stoneman, who had been considered the frontrunner to replace Ballance following the diagnosis of a broken left index finger, sustained while batting in the second innings at Trent Bridge last week.
Stoneman, who moved to Surrey from Durham over the winter, has been averaging 58.53 from eight Championship matches, including three centuries with a high score of 197. At 30, he has had time to groove a well-rounded game, but perhaps tellingly, the England coach Trevor Bayliss has never yet seen him bat in the flesh.
England could feature as many as three debutants at The Oval, which will be hosting its 100th Test, if they opt to give a maiden cap to the Middlesex seamer, Toby Roland-Jones, who has been retained in the squad after going unused in the first two Tests of the summer. He may be considered an option ahead of Durham's Mark Wood, who sustained a bruised heel in a sub-par performance at Trent Bridge.
England will report to The Oval on Tuesday, where they will train in the afternoon, having taken a few days off in the wake of their crushing 340-run defeat in the second Test.