NEW YORK (TheStreet) -- UnitedHealth Group (UNH) has reportedly approached Aetna (AET) about a takeover deal valued at more than $40 billion, according to the Wall Street Journal, one of many rumors amid a wave of speculation about consolidation among the big five managed-care companies.

With Aetna and other insurers said to be interested in acquiring Humana (HUM) and Anthem (ANTM) in talks to buy smaller rival Cigna (CI), Vishnu Lekraj, an equity analyst at Morningstar, said the likelihood of consolidation is very high, as these companies need to build scale and increase memberships in order to survive during the coming years.

In terms of UnitedHealth and Aetna, Lekraj finds it a bit surprising that UnitedHealth is in the mix, because it is the largest insurer in the U.S. with some of the most diversified memberships. Lekraj doesn't see the company under pressure to make a move now.

He acknowledges, however, that UnitedHealth could face challenges down the road if it doesn't make a move, especially with Anthem looking to buy one of the other three smaller managed-care companies. Lekraj believes the bigger the compan, the better off it will be. Lekraj said Aetna's commercial membership base would fit in nicely with UnitedHealth's membership base of more Medicare Advantage customers.

As for investors, Lekraj warns to be cautious amid stock-price volatility and lofty valuations. He believes investors should look for the companies that should thrive longer term, namely Anthem and UnitedHealth Group.

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