After six months out with a foot injury Haskell played almost an hour in Wasps' Champions Cup win at Zebre on Sunday.
He reported minor soreness in his foot after the game, but still expected to attend the training camp as planned.
Back-row forward Jack Clifford will also remain at his club Harlequins.
He picked up an injury in the Challenge Cup defeat by Stade Francais in Paris, having only recently returned from ankle surgery.
It is unclear when Haskell, who has 70 caps, will be able to join up with the England squad. He will stay at Wasps for treatment for the time being.
"Both players sustained injuries in their club games on Sunday and were due to arrive in Vilamoura today but will instead stay in the UK to undergo further medical assessments after which a decision will be made on when they join up with England," the RFU said.
England's first match in the tournament is against France on 4 February.
Flanker James Haskell's hopes of playing in England's Six Nations opener against France have been dealt a blow, after it was revealed he would not yet be joining the squad in Portugal.