Once upon a time - in his first spell at Atl. Madrid - Griezman was one of the best forwards in the world, so not sure why you bring him into the "equation" of Sissoko & Rabiot and caps (btw, in France they don't get caps when they play for the national team, so theoretically he's got 0 caps

). He certainly deserved all his caps.
Right now, yes - with him being 31 and in decline - he would most probably be a bad signing for Man Utd. But not necessarily worse than Rabiot or any other of their present targets.