Morsi’s sweeping change of military leadership — sacking the defense minister and service chiefs who had been appointed by former president Hosni Mubarak — appears to have taken the United States by surprise. But officials weren’t ringing alarm bells Sunday night, cautioning that this is in part a generational change, replacing figures who had become increasingly unpopular and isolated in post-revolutionary Egypt.

U.S. officials specifically discounted rumors that were circulating late Sunday that Sissi is an Islamist with secret connections to the Muslim Brotherhood. To the contrary, officials say, Sissi is well known to the U.S. military after spending a year of professional training in the United States and was regarded as a generally effective head of military intelligence.

What’s indisputable is that the Muslim Brotherhood, of which Morsi is a longtime member, has now tightened its grip on Egypt, controlling the military as well as the presidency and the parliament. That’s either an example of democracy in action and civilian control of the military, or a Muslim Brotherhood putsch, depending on your viewpoint. It probably has elements of both.

The U.S. view is that the replacement of aging top military leaders, in itself, isn’t worrying. But they would be concerned if Morsi moved to make changes in Egypt’s judiciary, which has been an important independent center of power since the Tahrir Square revolution that deposed Mubarak in February 2011. Worries about the judiciary were prompted by another Morsi move Sunday — to appoint senior judge Mahmoud Mekki as vice president. The fear is that Mekki, as a former jurist, might reject rulings by the courts.

Field Marshall Mohammed Hussein Tantawi, who was fired as defense minister, had become a symbol of the remoteness, and often, incompetence, of the military body, known as the Supreme Council of the Armed Forces, or SCAF, that ruled until Morsi’s inauguration. Its days were clearly numbered, but Morsi’s moves have come with the suddenness of a coup.

U.S. officials don’t appear to have evidence that the purge was planned or debated by top leadership of the Muslim Brotherhood. Instead, Morsi used the terrorist attack in Sinai last week that killed 16 Egyptian soldiers as an excuse for installing new leadership in the military. The first key change was Thursday’s firing of the intelligence chief, Gen. Murad Muwafi, who had won praise from U.S., Israeli and European officials — in part because he had been pressing for months for a crackdown against terrorist groups taking root in the Sinai.

Sissi was seen as a cooperative associate of Muwafi, though less involved in counter-terrorism. He has met with John Brennan, the White House counter-terrorism chief. “He’s a solid guy, who has shown a solid level of cooperation,” said one U.S. official of Sissi. Like Muwafi, Sissi had some contact with Israel in his role as head of military intelligence.

The Israelis are said to be more concerned by Sunday’s purge, worrying that Morsi is taking a series of steps that may be leading toward a collision with Jerusalem. But for both the United States and Israel, watching developments in Egypt is a bit like riding a tiger — potentially very dangerous, but impossible to steer.