Back-row forward Joe Worsley has been cut from England’s training squad and is likely to miss the World Cup.
"Thomas had an injury to his calf and he hasn't been able to train that much," BBC Sport quotes Johnson as saying. "He has gone back to Leicester to participate in their pre-season.
"The other decisions have all been tough because everyone has participated and contributed. It is particularly tough on Stretts because he has trained very, very well.
"Simpson-Daniel has had a couple of niggling injuries and a bit of illness.”
Worsley was plagued with injuries last season, but the Wasps player hasn’t made the cut despite his comeback.
"Joe has come back from some injuries last year and worked incredibly hard to get back in but has come up a little bit short right now."
England’s first World Cup meeting will be against Argentina on September 10.