49ers RB Christian McCaffrey dealing with Achilles tendinitis, Kyle Shanahan says

San Francisco coach Kyle Shanahan said All-Pro running back Christian McCaffrey could have played in the 49ers' opener if it had been a playoff game but the team held him out to make sure his Achilles tendinitis didn't become a more serious issue.

The decision to hold McCaffrey out of Monday night's 32-19 win over the New York Jets came as a surprise to many and upset many fantasy football players who had taken the star running back with one of the top picks in their drafts.

[Related: Niners rolling over Jets without Christian McCaffrey bad news for rest of NFL ]

McCaffrey had been listed as questionable on the injury report but said Friday he expected to play. Adding more confusion to the issue was a postgame interview on television by fill-in Jordan Mason, who said he was told Friday that he would get the start.

Shanahan said the decision was not made until Monday and that he only told Mason to prepare to possibly have a heavier load with McCaffrey dealing with the injury.

The NFL routinely looks into possible violations of injury reporting and can issue fines if the league determines a team subverted the process.

A league spokesman declined to comment Tuesday on the specifics of McCaffrey's situation.

Shanahan said Tuesday that McCaffrey is still day to day but that the decision to sit him during the opener was an easy one. McCaffrey got injured early in training camp and missed four weeks of practice before returning to the field last week.

"The Achilles is tendinitis and that stuff comes and goes." Shanahan said. "When it is acting up, it's something you've got to be very careful about. Christian's very diligent about that stuff. If it was a playoff game, he made it very clear to me he believed he could go.