The Holland international missed the last five games of the season after dislocating his shoulder in the 3-0 defeat to Sunderland on 14 April.
He underwent an operation a few days later and has since stepped up his rehabilitation in a bid to be fit for pre-season.
"Just finished my first proper gym session of the new season. Shoulder is coming on well. Can't wait for season to start," tweeted the 25-year-old.
Steve Harper Rob Elliot deputised when Krul was unavailable or rested last term, but the Harper will formally leave the club after 20 years on July 1 when his contract expires, leaving Elliot and youngster Jak Alnwick to provide the back-up.