Law missed out on his first league game of the season against Peterborough United last time out after tweaking his knee in the recent draw with MK Dons.
McCall reckons Law could be ruled out for possibly another week, meaning the midfielder is also a doubt for next Tuesday's away match at Charlton Athletic.
Stephen Darby is likely to be back in contention for Saturday's match at the Ricoh Arena, however, following a nerve problem in his arm.
McCall said: "Nicky will probably be a bit longer than we first thought - maybe another week.
"He still has a little niggle, but it is nothing sinister. He will certainly be out for this Saturday (vs Coventry City) though.
"Darbs (Stephen Darby) is back and fine, which is clearly good news."