United are seen as the favourites to win the trophy this weekend, but Nicholas has claimed that the Red Devils "have been average" this term, and has backed Chelsea to land the crown.
"I think the bookies are slightly edging towards Manchester United and that is perhaps due to Alexis Sanchez's record at Wembley, otherwise I don't really see them as favourites," Nicholas told Sky Sports News.
"United have been average, Chelsea have been unpredictable... I've never heard Jose Mourinho say second place is a good season and I think he could end the season empty handed as well."
United recorded a 2-1 win when the two teams last locked horns in the Premier League in February.