For Johnson, it appears to have been illness.
[Johnson] was taken ill at the Etihad Stadium on Saturday.
Johnson, meanwhile, received medical treatment in the dressing room at the Etihad after being sick following his substitution.
The 25-year-old was subdued against his former employers on Saturday and was clearly out of sorts.
Manager Martin O’Neill said: "I hadn’t realised that he had a problem until my attention was drawn to the fact he was signalling to come off.
"Clearly he had a problem, though.
"He was feeling unwell and was vomiting in the dressing room afterwards and had to be attended to by the medical people."
McClean, who has had differences (to put it nicely) with the Ireland setup recently, seems to be the more serious issue.
McClean could miss out on the Republic of Ireland’s World Cup qualifiers against Germany and the Faroe Islands after suffering a tightening in his groin.
The former Derry City winger was due to undergo a scan on his groin today
Here's more from SBNation's Sunderland blog, The Roker Report.
Looking into the injury further, it's slightly worrying news for us. On the positive side, a grade one strain is the lowest of all the possible grades given to such injuries. On the negative side, others who have suffered such injuries usually take anywhere between one and three weeks to recover from it.
With him withdrawing from the squad to play this Friday and the following Tuesday, it's clearly been deemed that he will take a solid ten days to recover already, and any more on top of that would certainly rule him out of the big derby game the following weekend.