Marsh arrived in Johannesburg just before 4am on Sunday (1pm AEDT) after one of the most significant 24-hour periods in his cricketing career.

On Friday evening he made an unexpectedly early comeback from a calf injury considered serious enough to cost him his spot in Australia's Test tour of South Africa. His unbeaten 63 was a key component in Perth winning its first Big Bash League final in three attempts.

The following evening, as his younger brother Mitch watched their sister Melissa, 28, play her final home match in her top-level basketball career, Shaun was an absentee, by virtue of notification that the South Africa tour duty that had been called off for him was suddenly back on, and that he needed to pack and get to the airport for a 11.45pm flight from Perth to Johannesburg.

"It was a bit frantic. I think I had about two hours to get home and get myself ready and be at the airport," Marsh told Cricket Australia TV on his arrival in Johannesburg, with his baggy-green cap in tow. "I got all my cricket gear out of the WACA [left there after the BBL final]."

The trigger for the selectors' change of heart about Marsh, who has not played Tests since a horror stint two years ago, was a combination of his strong showing in the BBL final and, more significantly, Shane Watson's inability to pass a fitness test. Watson has a calf injury, the same reason Marsh was ruled out for on January 30.

Watson injured his right calf a week earlier, but the team elected to keep that quiet and nurse him through in the expectation it would heal well before the start of Wednesday's Test. The first acknowledgement of a possible problem was on Friday when the team said Watson, who had not bowled at all during the past week, would not be bowling in its intra-squad match because of calf "tightness".

On Saturday, the hierarchy quietly put him through a fitness test and found the irritation was bad enough to stop the 32-year-old from either bowling or batting over the next week.

"We were hopeful he'd recover, but it needed to be a little bit more advanced than what it is at the moment," team physio Alex Kountouris said on Saturday in announcing Watson had already been ruled out of the first Test. "He's had these [type of injuries] before . . . they can take a short period of time or they can taken an extended period of time. In this case it's looking like it's going to be a little bit longer."

Watson's bid to regain fitness during the series will be hampered by the tight schedule, with only three days between the first and second Tests and a futher four between the second and third. Furthermore, Kountoutris conceded it was likely there would be a four-day or five-day "lag" between Watson being fit to bat and being fit to bowl.

"There is usually a bit of a lag, because running [in the field] and running between the wickets is generally a bit easier than bowling," he said. "Hopefully he gets to the point where he's able to train unrestricted two or three days out [from the Test], which is what we were planning to do here, and then being able to reproduce that two or three times and be confident he can get through a game."

The timing of Marsh's arrival is, despite the haste, quite favourable to him and his chances of playing on Wednesday, as Sunday had already been designated as a rest day for the Australian squad. Marsh's first opportunity to train with his teammates will be on Monday morning at Centurion Park, the venue of the first Test. He said he was rapt to be back in Test selection contention.

"Obviously it's been pretty crazy. It was nice to get the phone call from Gavin [Dovey, team manager]. To be over here now is fantastic. I can't wait to meet up with the boys and get started," he said.

"I'm just really excited now to be part of a Test tour and be with the boys again.

"It will be good to meet up with them today and get over a bit of jetlag, get some more sleep and be ready to go tomorrow at training."

Marsh and Alex Doolan were the two new batsmen originally chosen for the tour, with one to fill the vacancy created by the omission of George Bailey after the Ashes. While there are now two vacancies, due to Watson's withdrawal, there are four candidates to fill them, with the others being Phillip Hughes, the beneficiary of Marsh's withdrawal, and all-rounder Moises Henriques.

Doolan, the only one of the four yet to debut for Australia, batted at one-drop in the team's intra-squad match on Friday. He was also the part of the first pair - Chris Rogers was the other - asked to bat in centre-wicket practice at the Wanderers Stadium on Saturday, doing so for about an hour.

While Doolan and Marsh were selectors' top two choices for the tour, the pecking order has been complicated by Hughes easily top-scoring in the practice match with a resolute 83 from 150 balls. After that innings the 25-year-old, whose exclusion from the initial squad was criticised by many pundits and fans, said he had accepted he was behind Doolan in the selection pecking order.

"He [Doolan] was obviously selected before me, so with myself coming over late I totally understand that. I know where I stand. I'm just happy to be in the squad," Hughes said. "If I play or don't play, just to be here to help out the boys is a good thing."

Since then, however, Marsh has been recalled. If selectors select the West Australian ahead of Hughes, it could be justified from the perspective he was part of their orignial squad, and was only discarded because they were unduly pessimistic about his recuperation period. It would, however, also be another example of Hughes outscoring his rivals for selection - in the practice match, just like he had done in the Sheffield Shield - and still being passed over.

A few days ago it seemed the Australian selectors' biggest dilemma was about where Watson should slot in in the batting order. Three days out from a pivotal series against the world's best team, the pressure on them to make the best call for Australia's chances of victory has increased significantly.