The 23-year-old Croft, who joins clubmate Harry Ellis in the party for the tour to South Africa, has made 25 appearances for Leicester Tigers this season at flanker and lock, and earned his 13th cap for England at the end of the RBS 6 Nations Championship.
Quinlan was suspended for 12 weeks after he was found guilty of an act contrary to good sportsmanship under Law 10.4(k) during Munster's Heineken Cup semi-final defeat by Leinster and his appeal was last night turned down.
Lions head coach Ian McGeechan said: "We had to be fair and allow Alan Quinlan the opportunity to appeal his suspension. However, following the outcome of the appeal we have replaced him with Tom Croft who, it must be said, was very unlucky not to have been selected in the original tour party.
"Tom showed in his recent performances for Leicester following the tour party announcement that he could put aside his disappointment and perform at a very high level.
“He was on our selection reserve list and his continued good form has been rewarded with selection to the Lions squad for the tour of South Africa."
Tour manager Gerald Davies said: "Alan Quinlan's suspension is disappointing and he must be devastated. That said, the Lions management fully accepts the decision. The outcome necessitated the selection of a new player and Ian McGeechan and his fellow selectors had no hesitation in selecting Leicester's Tom Croft."